Why you have fight xemmas three times in Kingdom Hearts 2

You have to fight Xemnas three times in Kingdom Hearts II because most J-RPGs have at least two forms or more for there final bosses.
If you talking about in-game, then, Xemnas is fought three times because he had abosorbed Kingdom Hearts, which gave him more power, so he had the ability to continue fighting after the first fight and on and on.