Company: MYI
Filing Date: 2025-09-08
Form Type: DEF 14A
Source: 0001193125-25-198172
Chunk: 29

Company: BLACKROCK MUNIYIELD QUALITY FUND III, INC.
Filing Date: 2025-09-08
Form: DEF 14A
Chunk 29
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 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

iv

PROXY STATEMENT

Dated September 8, 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 Proxy Statement is furnished to you as a preferred 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 “Target Fund” or collectively as the “Target Funds.” The proxies
will be voted at the joint special meeting of the shareholders of each Fund and at any and all adjournments, postponements and delays thereof (the “Special Meeting”). The Special Meeting will be held on October 15, 2025 at 1:00 p.m.
(Eastern Time) to consider the proposals set forth below and discussed in greater detail elsewhere in this Proxy Statement. The Special Meeting will be held in a virtual meeting format only. Share