I dunno...I'm thinking 3 partners is a little much, but it would be pretty awesome. Might make the fights a little too easy, though.
I think it would be cool to have that one Merlon guy as a partner. Probably the one from the first Paper Mario game, since you got to know him better. He could use heal, some sort of offensive blast thing, star storm, increasing stats power, I don't know. I just think he'd be a pretty awesome partner.