ff12 question :)

So, I'm back playing FF12, and I have a leveling question for all of you that have played it. I just finished the Dreadknot Leviathan section, after doing some leveling in the Lhasa Mines. So my characters are all around Level 20 except for Vaan, who's at Level 12... :X Is it possible to play the game only leveling three characters, or do I want to have a balanced group? My favored party right now is Basch, Balthier, and Fran, although I think I may switch Fran out for Ashe or Penelo...