I didn't think part iii sucked. Casting Sophia wa .... a bad move. But it closed the deal. Michael couldn't give up power without doing one more deal, and it cost him his last chance to reconcile with Kate and his son, and got his daughter killed. And remember Kay had had an abortion that ended her pregnancy with what would have been Michael's second son.
It was slow. And the action almost got in the way of watching Pacino destroy his character.