Fr tho this is how it should have ended
In some sort of final shootout, Trevor sacrifices himself to save the other two, displaying how despite all the wrongdoings and hate he has for michael, part of him still loves him like a brother and he's ultimately willing to die for him. This saves their lives, but to little avail as Michael is then captured by authorities and sentenced to life in prison. His wife marries some other rich jerk whilst he loses everything and is stuck watching life go on without him. Franklin is the only one who manages to escape, he takes his share of the loot and flees to South East Asia, settling down on a small island paradise where he spends his days lounging on the beach sipping cocktails, resembling Michael after he retired. But it's not much of a victory, the money doesn't matter, his mind is somewhere else. He yearns for the thrill and excitement of a criminal lifestyle, it's the only thing that will satisfy him now. And so when Lester calls him, talking about another job he needs him for, Franklin accepts.