Part III strains my credibility filters. Michael has killed or directed the killing of people every three minutes in Part I and II. He also had to testify in Congress. Early in Part III he is being accepted into a Catholic order and somehow his massive family fortune appears to be a do gooder organization.
The helicopter machine gun scene where the Dons are nearly totally wiped out is in sharp contrast with the rest of the movie, but at least it makes it congruent with part I and II.
So much for the do gooder section of the movie, I will not give away the plot, though Wikipedia does, but are pretty much back to blood and gore.