The 300th anniversary that follows America250 in 2076 is called the United States Tricentennial.
A tricentennial marks 300 years. Since America250 commemorates the 250th anniversary in 2026, adding another 50 years places the next major milestone in 2076. The term follows the same numerical naming pattern as bicentennial for 200 years and semiquincentennial for 250 years.
The 2076 milestone will also coincide with the 300th anniversary of the Declaration of Independence’s adoption in 1776. The United States’ centennial occurred in 1876, its sesquicentennial in 1926, its bicentennial in 1976, and its semiquincentennial in 2026.
The word is sometimes confused with “tercentennial,” another accepted term for a 300th anniversary. For the United States anniversary sequence, however, the Semiquincentennial article identifies the next observance as the Tricentennial.