In general, prequel to any film is inherently doomed to fail as it competes with an imagination of every person that saw the original film.
The Good, The Bad, and the Ugly and The Godfather 2 say hi.
The Dollars Trilogy seems so unrelated to each other that you cannot even consider them sequel/prequel with respect to each other.
As for Godfather 2, I actually dislike the backstory part of the film.