Kurogane: He'd be born in the Earth kingdom, but he wouldn't be a bender. He'd rely on his physical strength and be a swordsman and probably wander around a lot. Sort of like Zuko in "Zuko Alone", like a wandering samurai.
Watanuki: Probably a waterbender, though he'd be pretty frantic about it and not very fluid at all, like waterbending kinda requires. Water is the most changeable element, which is why it'd probably work for him. Fire'd be DEADLY (for Dou), earth is too solid and air isn't grounded enough. So I think water would fit him best. As he matures I think he'd get better at waterbending and eventually become all awesome and fluid and cool. But if this was an AU story I think he'd be totally spastic and fail at it for a while until he began to change.
Kentarou: Hmmm. I'd say airbender, but he doesn't seem disciplined or spiritual enough. So he'd probably just be a regular guy in the Earth Kingdom.