I'm a bit new here so I'm not sure if you guys are aware that the original script for GF 3, from May of 89, is viewable online. If you check it out you'll see that Tom was supposed to have a tremendous part. In this early draft it seems to me that Puzo/FFC had Hagen loyal to Michael to the very end. Anyway, you can see it at joblo.com/movie scripts Just click on The Godfather 3 link.
Went and read it; very interesting.
As I suspected, the Vincent/Mary romance was a much smaller part of the storyline.
