Spoiler: Omnislash COULD kill Sephiroth, but if you leave it, Sephiroth attacks you, then you counter and kill him in one hit. So Sephiroth would lose straight away if it was a three way match. I'm voting that Zack would win, because he'd probably have the same limit breakers as Cloud, but they'd be better, as Cloud probably stole Zacks limit breaks, as well as memories.