Company: MYI
Filing Date: 2025-09-05
Form Type: 424B3
Source: 0001193125-25-196285
Chunk: 349

Company: BLACKROCK MUNIYIELD QUALITY FUND III, INC.
Filing Date: 2025-09-05
Form: 424B3
Chunk 349
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5% of the Fund’s total assets would be invested in securities of that issuer or (b) the Fund would hold more than 10% of the outstanding voting securities of that issuer. For purposes of this restriction, the Fund will
regard each state and each political subdivision, agency or instrumentality of such state and each multi-state agency of which such state is a member and each public authority which issues securities on behalf of a private entity as a separate
issuer, except that if the security is backed only by the assets and revenues of a non-government entity then the entity with the ultimate responsibility for the payment of interest and principal may be
regarded as the sole issuer. With respect to the remaining 25% of its total assets, the Fund can invest more than 5% of its assets in one issuer. Under the 1940 Act, a fund cannot change its classification from diversified to non-diversified without shareholder approval.

In order to obtain and maintain the
desired ratings for the VMTP Shares from Moody’s and Fitch and to comply with the Eligible Assets requirement of the Articles Supplementary, the Fund will be required to comply with investment quality, diversification and other guidelines
established by Moody’s and Fitch. Such guidelines will likely be more restrictive than the restrictions set forth above.

B-6

MYI_PROXY_0825

STATEMENT OF ADDITIONAL INFORMATION

BLACKROCK MUNIVEST FUND II, INC.

BLACKROCK MUNIYIELD MICHIGAN QUALITY FUND, INC.

BLACKROCK MUNIVEST FUND, INC.

BLACKROCK MUNIYIELD QUALITY FUND III, INC.

Dated September 4, 2025

This Statement of Additional Information is available to the common shareholders of BlackRock MuniVest Fund II, Inc. (NYSE Ticker: MVT)
(“MVT”), BlackRock MuniYield Fund, Inc. (NYSE Ticker: MIY) (“MIY”), BlackRock MuniVest Fund, Inc. (NYSE Ticker: MVF) (“MVF”), in connection with the Reorganization (each, a “Reorganization” and
collectively, the “Reorganizations”) of each of MVT, MIY and MVF into BlackRock MuniYield Quality Fund III, Inc. (NYSE Ticker: MYI) (“MYI” or the “Acquiring Fund,” and collectively with MVT, MIY and MVF,