Part 3 would have been wonderful if Pacino's ego hadn't contributed to his overacting in 3. He received such acclaim for Parts 1 and 2 in the 16-year interval between 2 and 3 and became so rich and famous, that he forgot how tightly-controlled Michael's original character had been.
The story line of 3 was very prescient, seeming to see the future about the Vatican banking scandal and the suspicious death of a pope. And the scenery and music were beautiful.
I think the long break between 2 and 3 contributed to the perception that it was not as good, because without a deep refresher course in the first two, the third was harder to understand.
Da Gunna kill da Pope!