Company: MYI
Filing Date: 2025-09-05
Form Type: 424B3
Source: 0001193125-25-196285
Chunk: 3

Company: BLACKROCK MUNIYIELD QUALITY FUND III, INC.
Filing Date: 2025-09-05
Form: 424B3
Chunk 3
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 in the postage-paid 
 envelope; or                                                                                               |

| • |     | By participating at the Special Meeting as described above. |

If you do not vote using one of the methods described above, you may be called by Georgeson LLC, the Funds’ proxy solicitor, to vote your shares. If you have any questions about the proposals to be voted on or the virtual Special Meeting, please call Georgeson LLC, the firm assisting us in the solicitation of proxies, toll free at 1-888-686-6309. As always, we appreciate your support. Sincerely, JOHN M. PERLOWSKI President and Chief Executive Officer of the Funds

Please vote now. Your vote is important.

To avoid the wasteful and unnecessary expense of further solicitation(s), we urge you to indicate your voting instructions on the enclosed proxy card, date and sign it and return it promptly in the postage-paid envelope provided, or record your voting instructions by telephone or via the internet, no matter how large or small your holdings may be. If you submit a properly executed proxy but do not indicate how you wish your common shares to be voted, your common shares will be voted “FOR” the proposal. If your common shares are held through a broker, you must provide voting instructions to your broker about how to vote your common shares in order for your broker to vote your common shares as you instruct at the Special Meeting.

September 4, 2025 IMPORTANT NOTICE TO COMMON SHAREHOLDERS OF BLACKROCK MUNIVEST FUND II, INC. BLACKROCK MUNIYIELD MICHIGAN QUALITY FUND, INC. BLACKROCK MUNIVEST FUND, INC. BLACKROCK MUNIYIELD QUALITY FUND III, INC. QUESTIONS & ANSWERS Although we urge you to read the entire Joint Proxy Statement/Prospectus, we have provided for your convenience a brief overview of some of the important questions concerning the joint special shareholder meeting (the “Special Meeting”) of BlackRock MuniVest Fund II, Inc. (“MVT”), BlackRock MuniYield Michigan Quality Fund, Inc. (“MIY”) and BlackRock MuniVest Fund, Inc. (“MVF”) (collectively, the “Target Funds”) and BlackRock MuniYield Quality Fund III, Inc. (“MYI” or the “Acquiring Fund” and collectively with MVT, MIY and MVF, the “Funds,” and each, a