Roger Ebert Remarked in his Review of GF III:

"Some of the dialogue scenes, especially in the beginning, sound vaguely awkward; the answers do not fit the questions, and conversations seem to have been rewritten in the editing room. Other shots - long shots, into the light, so we cannot see the characters' lips - look suspiciously like scenes that were filmed first and dubbed later. The whole ambitious final movement of the film - in which two separate intrigues are intercut with the progress of an opera being sung by Anthony - is intended to be suspenseful but is so confusing, we are not even sure which place (Sicily, Rome, London?) one of the intrigues is occurring. The final scene of the movie, which is intended to echo Marlon Brando's famous death scene, is perfunctory and awkward.

"And yet it's strange how the earlier movies fill in the gaps left by this one, and answer the questions. It is, I suspect, not even possible to understand this film without knowing the first two, and yet, knowing them, "Part III" works better than it should, evokes the same sense of wasted greatness, of misdirected genius. Both Don Vito Corleone and Don Michael Corleone could have been great men. But they lacked that final shred of character that would have allowed them to break free from their own pasts. Or perhaps their tragedies were dictated by circumstances. Perhaps they were simply born into the wrong family. "

I think he hit the nail on the head with this one.