the ending(s) to your trilogy should not:
-Depend on the Endingtron 3000 right at the last minute.
-Be variants of the same goddamned thing.
-UNDO ALL THE HARD WORK YOU DID THROUGHOUT THE WHOLE GODDAMNED SERIES.
-Depend on your actions throughout the trilogy, or at the very least the last game. (like silent hill or Shin Megami Tensei Nocturne)
-At the very least have significantly different epilogues.
-Have a Golden Ending as a reward for all of your hard work throughout the whole series. Go ahead and keep the whole Main Character's Noble Sacrifice thing for the sake of drama, but at least let the players know that they've made the world a better place.