One game at a time.
Game 5 is do or die, in Miami, with all the pressure on them. I doubt the Big 3 want to go out easy, and what I can guarantee is that they'll fight, and fight hard.
If they somehow grind out game 5 the series shifts back to Boston. Anything is possible after that.
The odds are stacked against them but if they somehow take Game 5 again, I think it'll lift everyone to maybe just maybe win the whole thing.