Sec Form 3 Filing - Comly Barbara J. @ MIAMI INTERNATIONAL HOLDINGS, INC. - 2025-08-13

FORM 3 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Comly Barbara J. 2. Issuer Name and Ticker or Trading Symbol MIAMI INTERNATIONAL HOLDINGS, INC. [ MIAX] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director _____ 10% Owner X __ Officer (give title below) _____ Other (specify below) EVP, GC & Corporate Secretary (Last) (First) (Middle) C/O MIAMI INTERNATIONAL HOLDINGS, INC., 7 ROSZEL ROAD, SUITE 1A 3. Date of Earliest Transaction (MM/DD/YY) 08/13/2025 (Street) PRINCETON, NJ08540 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 868,918 D

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Series B Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 100,500 D Nonvoting Common Stock ( 2 ) ( 2 ) ( 2 ) Common Stock 155,667 D Incentive Stock Option (Right to Buy) $ 12 ( 3 ) 08/02/2026 Nonvoting Common Stock 8,333 D Incentive Stock Option (Right to Buy) $ 12 ( 3 ) 05/28/2028 Nonvoting Common Stock 16,666 D Incentive Stock Option (Right to Buy) $ 12 ( 3 ) 07/31/2029 Nonvoting Common Stock 8,333 D Incentive Stock Option (Right to Buy) $ 15.22 ( 3 ) 01/27/2031 Nonvoting Common Stock 13,140 D Incentive Stock Option (Right to Buy) $ 19.84 ( 4 ) 03/28/2033 Common Stock 10,080 D Incentive Stock Option (Right to Buy) $ 20.08 ( 5 ) 06/09/2034 Common Stock 4,980 D Incentive Stock Option (Right to Buy) $ 22.4 ( 6 ) 06/15/2035 Common Stock 4,464 D Nonqualified Stock Option (Right to Buy) $ 12 ( 7 ) 08/02/2026 Nonvoting Common Stock 141,667 D Nonqualified Stock Option (Right to Buy) $ 12 ( 7 ) 05/28/2028 Nonvoting Common Stock 133,334 D Nonqualified Stock Option (Right to Buy) $ 12 ( 7 ) 07/31/2029 Nonvoting Common Stock 141,667 D Nonqualified Stock Option (Right to Buy) $ 15.22 ( 7 ) 01/27/2031 Nonvoting Common Stock 136,860 D Nonqualified Stock Option (Right to Buy) $ 16.14 ( 7 ) 09/09/2031 Nonvoting Common Stock 150,000 D Nonqualified Stock Option (Right to Buy) $ 19.84 ( 8 ) 03/28/2033 Common Stock 114,920 D Nonqualified Stock Option (Right to Buy) $ 20.08 ( 9 ) 06/09/2034 Common Stock 95,020 D Nonqualified Stock Option (Right to Buy) $ 22.4 ( 10 ) 06/15/2035 Common Stock 70,536 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Comly Barbara J. C/O MIAMI INTERNATIONAL HOLDINGS, INC. 7 ROSZEL ROAD, SUITE 1A PRINCETON, NJ08540 EVP, GC & Corporate Secretary Signatures /s/Alessandra Maria Corona Henriques, Attorney-in-Fact 08/13/2025 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The shares of Series B Preferred Stock are convertible on a one-for-one basis into shares of common stock commencing on the Company's initial public offering of its common stock ("IPO") effective date, which is August 13, 2025. The Series B Preferred Stock has no expiration date. ( 2 )The shares of nonvoting common stock are convertible on a one-for-one basis into shares of common stock commencing at any time. The nonvoting common stock has no expiration date. ( 3 )The incentive stock options are fully vested. The options were granted initially as the right to buy nonvoting common stock. On the closing date of the IPO, these will convert to the right to buy common stock instead of nonvoting common stock. ( 4 )5,040 of the shares subject to this incentive stock option are vested and the remaining 5,040 will vest on March 29, 2026 subject to the reporting person's continued service to the Issuer or its subsidiaries through the applicable vesting date. ( 5 )All of the shares subject to this incentive stock option will vest on June 10, 2027, subject to the reporting person's continued service to the Issuer or its subsidiaries through the applicable vesting date. ( 6 )All of the shares subject to this incentive stock option will vest on June 16, 2028, subject to the reporting person's continued service to the Issuer or its subsidiaries through the applicable vesting date. ( 7 )The nonqualified stock options are fully vested. The options were granted initially as the right to buy nonvoting common stock. On the closing date of the IPO, these will convert to the right to buy common stock instead of nonvoting common stock. ( 8 )78,294 of the shares subject to this nonqualified stock option are vested and the remaining 36,626 will vest on March 29, 2026 subject to the reporting person's continued service to the Issuer or its subsidiaries through the applicable vesting date. ( 9 )33,334 of the shares subject to this nonqualified stock option are vested, 33,333 will vest on June 10, 2026 and the remaining 28,353 will vest on June 10, 2027, subject to the reporting person's continued service to the Issuer or its subsidiaries through the applicable vesting date. ( 10 )25,000 of the shares subject to this nonqualified stock option will vest on June 16, 2026, 25,000 will vest on June 16, 2027 and the remaining 20,536 will vest on June 16, 2028, subject to the reporting person's continued service to the Issuer or its subsidiaries through the applicable vesting date.

Remarks: See Exhibit 24.1 - Power of Attorney

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

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