In a sense GF II wasn’t a sequel. Rather, it picked up the parts of the GF I novel that weren’t included in the original movie, completing the basic story. GF III was really the sequel and it was horrible compared to the other two.
Godfather III is actually a good movie. The story is quite brilliant, and the relationship between Kay and Michael beautifully rendered. But there are a couple of casting miscues, and - when following in the footsteps of two of the greatest movies ever made - “good” ain’t good enough.