UNAUDITED PRO FORMA CONDENSED COMBINED FINANCIAL INFORMATION RELATING TO THE MERGER WITH LEGACYTEXAS FINANCIAL GROUP, INC.

Exhibit 99.3

UNAUDITED PRO FORMA CONDENSED COMBINED FINANCIAL INFORMATION RELATING TO

THE MERGER WITH LEGACYTEXAS FINANCIAL GROUP, INC.

On November 1, 2019, pursuant to the Agreement and Plan of Reorganization, dated as of June 16, 2019 (the “Merger Agreement”), by and between LegacyTexas Financial Group, Inc., a Maryland corporation (“LegacyTexas”), and Prosperity Bancshares, Inc., a Texas corporation (“Prosperity”), LegacyTexas merged with and into Prosperity (the “Merger”), with Prosperity as the surviving entity in the Merger. Immediately after the Merger, LegacyTexas’ wholly owned bank subsidiary, LegacyTexas Bank, merged with and into Prosperity’s wholly owned bank subsidiary, Prosperity Bank (the “Bank Merger”), with Prosperity Bank as the surviving entity in the Bank Merger.

Subject to the terms and conditions set forth in the Merger Agreement, at the effective time of the Merger (the “Effective Time”), each share of common stock, par value $0.01 per share, of LegacyTexas (“Legacy Common Stock”) (subject to certain customary exceptions, which were cancelled in the Merger) was converted into the right to receive (i) 0.5280 (the “Exchange Ratio”) shares of common stock, par value $1.00 per share, of Prosperity (“Prosperity Common Stock”); and (ii) $6.28 in cash, without interest (“Per Share Cash Consideration”) (the consideration described in the foregoing clauses (i) and (ii), the “Merger Consideration”). No fractional shares of Prosperity Common Stock were issued in the Merger, and LegacyTexas stockholders became entitled to receive cash in lieu of fractional shares.

At the Effective Time, subject to the terms and conditions of the Merger Agreement, each option granted by LegacyTexas to purchase shares of Legacy Common Stock under the LegacyTexas equity compensation plans fully vested and was cancelled and converted into the right to receive the Merger Consideration with respect to a number of shares of Legacy Common Stock (rounded down to the nearest whole share) equal to the quotient of (x) the product of (A) the number of shares of Legacy Common Stock subject to such option multiplied by (B) the excess, if any, of (i) the sum of the Per Share Stock Consideration (as defined in the Merger Agreement) and the Per Share Cash Consideration over (ii) the exercise price per share of Legacy Common Stock under the option, divided by (y) the sum of the Per Share Stock Consideration and the Per Share Cash Consideration. At the Effective Time, subject to the terms and conditions of the Merger Agreement, each LegacyTexas restricted stock award and performance share award was converted into the right to receive the Merger Consideration (with any performance-based vesting conditions applicable to such awards immediately prior to the Effective Time deemed satisfied at target performance levels) with respect to the number of shares of Legacy Common Stock subject to such restricted stock award or performance share award.

Prosperity has prepared the unaudited pro forma condensed combined statements of income appearing below to present on a pro forma basis the consolidated statements of income of Prosperity assuming that the Merger was consummated on January 1, 2018, and to provide information with respect to the pro forma consolidated results of operations that Prosperity would have had for the year ended December 31, 2018, and the six months ended June 30, 2019. Prosperity has prepared the unaudited pro forma condensed combined balance sheet appearing below to present on a pro forma basis the consolidated financial position of Prosperity assuming that the Merger was consummated on June 30, 2019.

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The unaudited pro forma condensed combined financial statements contain information derived from, and should be read together with:

The Merger and Bank Merger are being accounted for as acquisitions of LegacyTexas and LegacyTexas Bank by Prosperity and Prosperity Bank, respectively, under the acquisition method of accounting in accordance with the Financial Accounting Standard Board’s Accounting Standard Codification Topic 805, Business Combinations (“ASC 805”). The unaudited pro forma condensed combined financial statements of Prosperity and the other pro forma combined financial information appearing below have been prepared using the acquisition method of accounting.

The unaudited pro forma condensed combined statements of income for the year ended December 31, 2018 and for the six months ended June 30, 2019 present the consolidated results of operations giving pro forma effect to the following as if they had occurred as of January 1, 2018:

The unaudited pro forma condensed combined balance sheet as of June 30, 2019 presents the consolidated financial position giving pro forma effect to the following as if they had occurred as of June 30, 2019:

The preparation of the unaudited pro forma condensed combined financial statements and related adjustments required Prosperity’s management to make certain assumptions and estimates, which it believes are reasonable under the circumstances. Prosperity has not had sufficient time to completely evaluate the significant identifiable long-lived tangible and identifiable intangible assets of LegacyTexas. Accordingly, the unaudited pro forma adjustments, including the allocations of the purchase price, are preliminary and have been made solely for the purpose of providing unaudited pro forma condensed combined financial information. Certain reclassifications have been made to the historical financial statements of LegacyTexas to conform to the presentation in Prosperity’s financial statements. Accordingly, the unaudited pro forma condensed combined financial statements and other unaudited pro forma condensed combined financial information are presented for illustrative purposes only and are not necessarily indicative of the results that might have occurred had the Merger taken place on January 1, 2018, for

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statement of income purposes, and on June 30, 2019, for balance sheet purposes. Historical results for any prior period are not necessarily indicative of results to be expected in any future period, and historical results for the six months ended June 30, 2019, are not necessarily indicative of results to be expected for all of 2019. A final determination of the fair values of LegacyTexas’ assets and liabilities will be based on the actual net tangible and intangible assets of LegacyTexas that existed as of the date of completion of the Merger. Consequently, amounts preliminarily allocated to goodwill and identifiable intangibles could change significantly from those allocations used in the unaudited pro forma condensed combined financial statements presented below and could result in a material change in amortization of acquired intangible assets.

In connection with the integration of the operations of Prosperity and LegacyTexas following the Effective Time, Prosperity anticipates that nonrecurring charges, such as costs associated with systems implementation, severance, and other costs related to exit or disposal activities, could be incurred. Prosperity is not yet able to fully determine the timing, nature and amount of these charges. However, these charges could affect the results of operations of the combined company in the period in which they are recorded. The unaudited pro forma condensed combined financial statements do not include the effects of the costs associated with any restructuring or integration activities resulting from the transaction, as they are nonrecurring in nature and not factually supportable at the time that the unaudited pro forma condensed combined financial statements were prepared. Additionally, the unaudited pro forma adjustments do not give effect to any nonrecurring or unusual restructuring charges that have been or may be incurred as a result of the integration of the two companies or any dispositions of assets resulting from such integration. Direct merger-related expenses estimated at $40.4 million are not included in the unaudited pro forma condensed combined statements of income.

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UNAUDITED PRO FORMA CONDENSED COMBINED BALANCE SHEET

AS OF JUNE 30, 2019

(in thousands)

The accompanying notes are an integral part of these unaudited pro forma condensed combined financial statements. Amounts may not add due to rounding.

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UNAUDITED PRO FORMA CONDENSED COMBINED STATEMENT OF INCOME

FOR THE SIX MONTHS ENDED JUNE 30, 2019

(in thousands, except per share data)

The accompanying notes are an integral part of these unaudited pro forma condensed combined financial statements. Amounts may not add due to rounding.

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UNAUDITED PRO FORMA CONDENSED COMBINED STATEMENT OF INCOME

FOR THE YEAR ENDED DECEMBER 31, 2018

(in thousands, except per share data)

The accompanying notes are an integral part of these unaudited pro forma condensed combined financial statements. Amounts may not add due to rounding.

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Notes to Unaudited Pro Forma Condensed Combined Financial Statements

Note 1. Basis of Pro Forma Presentation

The accompanying unaudited pro forma condensed combined financial statements and related notes were prepared in accordance with Article 11 of Regulation S-X. The unaudited pro forma condensed combined statement of income for the six months ended June 30, 2019 and for the year ended December 31, 2018 combine the historical consolidated statements of income of Prosperity and LegacyTexas giving effect to the merger as if it had been completed on January 1, 2018. The unaudited pro forma condensed combined balance sheet as of June 30, 2019 combines the historical consolidated balance sheets of Prosperity and LegacyTexas giving effect to the merger as if it had been completed on June 30, 2019.

Prosperity’s and LegacyTexas’ historical financial statements were prepared in accordance with U.S. GAAP and presented in U.S. dollars. As discussed in Note 4 and Note 5, certain reclassifications were made to align Prosperity’s and LegacyTexas’ financial statement presentation. Prosperity has not identified all adjustments necessary to conform LegacyTexas’ accounting policies to Prosperity’s accounting policies. Prosperity is in the process of performing a more detailed review of Legacy’s accounting policies. As a result of that review, differences could be identified between the accounting policies of the two companies that, when conformed, could have a material impact on the combined company’s financial information.

The accompanying unaudited pro forma condensed combined financial statements and related notes were prepared using the acquisition method of accounting under the provisions of ASC 805, with Prosperity considered the acquirer of LegacyTexas. ASC 805 requires, among other things, that the assets acquired and liabilities assumed in a business combination be recognized at their fair values as of the acquisition date. For purposes of the unaudited pro forma condensed combined balance sheet, the merger consideration has been allocated to the assets acquired and liabilities assumed of LegacyTexas based upon management’s preliminary estimate of their fair values as of June 30, 2019. Prosperity has not completed the valuation analysis and calculations in sufficient detail necessary to arrive at the required estimates of the fair market value of the LegacyTexas’ assets to be acquired or liabilities assumed, other than a preliminary estimate for intangible assets and held-to-maturity securities. Accordingly, apart from the aforementioned, certain LegacyTexas assets and liabilities are presented at their respective carrying amounts and should be treated as preliminary values. Any differences between the fair value of the merger consideration and the fair value of the assets acquired and liabilities assumed will be recorded as goodwill. Accordingly, the purchase price allocation and related adjustments reflected in these unaudited pro forma condensed combined financial statements are preliminary and subject to revision based on a final determination of fair value.

All dollar amounts and share amounts (except per share data) presented within these Notes to Unaudited Pro Forma Condensed Combined Financial Statements are in thousands.

Note 2. Estimated Operational Cost Savings

Prosperity anticipates operational cost savings in connection with the acquisition of LegacyTexas. Prosperity anticipates that these savings will occur through the combination of back office operations and elimination of duplicate general operations, administrative and salary and benefits expense. Estimated cost savings are not presented as part of the pro forma adjustments and there can be no assurance they will be achieved in the amount or manner currently contemplated.

Note 3. Anticipated Reduction in Fee Income

Prosperity anticipates loss of income related to reduced debit and ATM card income. The combined company will be subject to the ▇▇▇▇▇▇ Act which imposes limits on debit and ATM card income. Previously, LegacyTexas was not subject to the ▇▇▇▇▇▇ Amendment. Such amounts are not presented as part of the pro forma adjustments.

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Note 4. Preliminary Purchase Price Allocation

The following table summarizes the purchase price allocation to the estimated fair value of assets and liabilities assumed in the merger (in thousands, except per share data):

The merger consideration as shown in the table above is allocated to the tangible and intangible assets acquired and liabilities assumed of LegacyTexas based on their preliminary estimated fair values. As mentioned above in Note 1, Prosperity has not completed the valuation analysis and calculations in sufficient detail necessary to arrive at the required estimates of the fair market value of the LegacyTexas assets to be acquired or liabilities assumed, other than a preliminary estimate for intangible assets and held-to-maturity securities. Accordingly, apart from the aforementioned, certain assets acquired and liabilities assumed are presented at their respective carrying amounts and should be treated as preliminary values. The fair value assessments are preliminary and are based upon available information and certain assumptions, which Prosperity believes are reasonable under the circumstances. Actual results may differ materially from the assumptions within the unaudited pro forma condensed combined financial statements.

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The following table sets forth a preliminary allocation of the merger consideration to the fair value of the identifiable tangible and intangible assets acquired and liabilities assumed of LegacyTexas using LegacyTexas’ unaudited consolidated balance sheet as of June 30, 2019:

Note 5. Pro forma Adjustments and Assumptions

The following pro forma adjustments have been reflected in the unaudited pro forma condensed combined financial information. All adjustments are based on current assumptions and valuations, which are subject to change.

(a) This adjustment represents the cash consideration for LegacyTexas and estimated direct transaction costs calculated as follows:

(b) Estimated fair market value adjustment on securities held to maturity.

(c) Adjustment to total loans of $175 million to reflect preliminary estimated fair value adjustments to acquired loans of $175 million for credit quality. Also, adjustment to eliminate historical allowance for loan losses of $92.2 million to reflect acquired loans and leases at fair value.

(d) Adjustment to reflect the reclassification of LegacyTexas’ right-of-use asset from bank premises to other assets to conform with Prosperity’s financial statement presentation.

(e) Adjustment to goodwill based on the preliminary purchase price allocation as follows:

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(f) Adjustment to core deposit intangible (“CDI”) based on the preliminary purchase price allocation as follows:

(g) Adjustment to Prosperity and LegacyTexas shareholders’ equity based upon the following:

(h) Adjustment to interest income for accretion on LegacyTexas’ previously acquired loans.

(i) Adjustment to interest expense for amortization on LegacyTexas’ previously acquired time deposits.

(j) Adjustment to reflect the reclassification of safe deposit box income from customer service fees to other noninterest income to conform with Prosperity’s financial statement presentation.

(k) Adjustment to reflect the reclassification of LegacyTexas’ gain/loss on sale of ORE from noninterest income to noninterest expense to conform with Prosperity’s financial statement presentation.

(l) Adjustment to reflect the reclassification of LegacyTexas’ equipment and depreciation expense (on FF&E and data processing equipment) from other noninterest expense to net occupancy and equipment expense to conform with Prosperity’s financial statement presentation.

(m) Estimated core deposit intangible at 2.0% of the acquired non-time deposits based upon 10 year life using straight line amortization method, net of LegacyTexas’ historical CDI.

(n) Adjustment to reflect the net federal income tax effect of the pro forma adjustments using Prosperity’s statutory tax rate of 21.0%.

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