It really is a huge problem with GF III that Tom Hagen is not in it, because quite clearly, much of the plotline towards the end of GF II is intended to set up a potential conflict between Hagen and Michael later on, and then when we get to GF III, that all goes out the window and they just say, "Tom Hagen? Oh, yeah, him. He died."

FFC should have done whatever it took to get Duvall back in as Tom Hagen, or failing that, maybe even recast the part (Michael Moriarity would have been a possible alternate Hagen - see the pic).

IMO the conflict between Hagen and Michael was always intended by Puzo and FFC to be the main theme of GF III.