Company: MYI
Filing Date: 2025-09-05
Form Type: 424B3
Source: 0001193125-25-196285
Chunk: 36

Company: BLACKROCK MUNIYIELD QUALITY FUND III, INC.
Filing Date: 2025-09-05
Form: 424B3
Chunk 36
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, AS APPLICABLE.

IN ORDER TO AVOID THE ADDITIONAL EXPENSE OF FURTHER SOLICITATION, WE ASK THAT YOU MAIL YOUR PROXY CARD OR RECORD YOUR VOTING INSTRUCTIONS BY TELEPHONE OR VIA THE INTERNET PROMPTLY.

For the Board of each Fund,

JOHN M. PERLOWSKI

President and Chief Executive Officer of the Funds

September 4, 2025

YOUR VOTE IS IMPORTANT.

PLEASE VOTE PROMPTLY BY SIGNING AND RETURNING THE ENCLOSED PROXY CARD OR BY RECORDING YOUR VOTING INSTRUCTIONS BY TELEPHONE OR VIA THE INTERNET, NO MATTER HOW MANY SHARES YOU OWN.

IMPORTANT NOTICE REGARDING THE AVAILABILITY OF PROXY MATERIALS FOR THE JOINT SPECIAL MEETING OF SHAREHOLDERS TO BE HELD ON OCTOBER 15, 2025.

THE PROXY STATEMENT FOR THIS MEETING IS AVAILABLE AT:

HTTPS://WWW.PROXY-DIRECT.COM/BLK-34668

xx

JOINT PROXY STATEMENT/PROSPECTUS

Dated September 4, 2025

BLACKROCK MUNIVEST FUND II, INC.

BLACKROCK MUNIYIELD MICHIGAN QUALITY FUND, INC.

BLACKROCK MUNIVEST FUND, INC.

BLACKROCK MUNIYIELD QUALITY FUND III, INC.

100 Bellevue Parkway

Wilmington, Delaware 19809

(800) 882-0052

This Joint Proxy Statement/Prospectus is furnished to you as a common shareholder of BlackRock MuniVest Fund II, Inc. (NYSE
Ticker: MVT) (“MVT”), BlackRock MuniYield Michigan Quality Fund, inc. (NYSE Ticker: MIY) (“MIY”), BlackRock MuniVest Fund, Inc. (NYSE Ticker: MVF) (“MVF”) and BlackRock MuniYield Quality Fund III, Inc. (NYSE
Ticker: MYI) (“MYI” or the “Acquiring Fund,” and collectively with MVT, MIY and MVF, the “Funds,” and each, a “Fund”) in connection with the solicitation of proxies by each Fund’s Board of
Directors (each, a “Board,” the members of which are referred to as “Board Members”). Each of MVT, MIY and MVF may be referred to herein individually as a “