Both are classic. Overall I'd say 2 is better, although I feel at times the flashbacks take you out of things. Even though I love how many of the flashbacks mirror issues Michael is going through in the present.
3 would have been good if Duvall had come back. It was going to be about Tom and Michael facing off while Michael's son made his way through the political world. I assume it would have ended with Michael's son being accidentally killed, similar to what happened to his daughter in the film.