You raise a really good question, Signora. It's difficult to tell who contributed what to their collaboration. I offer an opinion and a guess: The fatal weakness of Part III, in my opinion, is the gaps in the plot, and the awkward dialog. These seem to me to match the gaps and dialog awkwardness in "Fools Die," "The Last Don" and "Omerta," three of Puzo's subsequent novels. I'd guess on that basis that Puzo must have had a big hand in writing Part III.