While it was mildly conspicous before, it's starting become somewhat of a theme for important things in this game to come in 3s.

Heartless, Nobodies, "Unbirths" (if that's what they end up being called)
Sora, Kairi and Riku
Mickey, Donald and Goofy (kind of)
Aqua, Terra, Ventus
Master Xehonart, Ansem (heartless), Xemnas
and now...
Roxas, Axel, Xion

There's little information, but as far as the scans and trailers are telling us these three seem to be a trio as well. I can't remember where I read this, but I think that one of the scans said that Xion is somehow connected to Kairi. Although I don't see how she would be Kairi's Nobody since Namine already exists, and since Namine and Xion are both together in a scene. While Axel has no known connection to Riku, it would make sense that Xion has some connection to Kairi since Roxas is involved.

What that connection is though, I have no thoughts on at the moment.