I think the first Godfather had many happy moments.
Things started to go downhill in the second part and Mike became a very lonley man.
But the end of part3? Was that the end of everything?
If that`s the case, it couldn`t be more depressing.

But this is so typical for gangster movies, not only the Godfather but also Godfellas and Scarface.
A happy beginning that ends in tragedy.