Yeah, but the Foundation trilogy has seven books.
The last two being prequels.

(Also it's connected to the Galactic Empire trilogy, which does have three books, but was published in reverse order, and the Robot series, a four book duology not to be confused with Asimov's other robot books, though it's set in the same universe, and also to The End of Eternity, which is set in a different timeline altogether but is sort of a prequel to the whole shebang.)