Imakuni? revealed to be an assist trophy.
And since they opted for the Generation 3 costumes I think this bumps down the chance for and 4th Gen Pokemon to be involved.
I imagine several options for the Final Smash:
1 A combined attack involving all 3 Pokemon
2 Each Pokemon has their own FS
3 Pokeballs rain from the sky, several Pokemon comeout and attack
4 Mewtwo (or other assorted legendary) comes to the playing field and executes a powerful attack
Of those I figure 2 and 4 to be the most viable. Each of those Pokemon have a powerful move exclusive to starter Pokemon (Blast Burn, Hydro Cannon, Frenzy Plant) which can be used based on which Pokemon got the FS item. Number 4 actually lets them drop Mewtwo from the roster spot, thus creating room for other characters but keeps the fans slightly happy (maybe you can even use Mewtwo for several seconds)
People on Smashboards did bring up some good points about the versitility of the 3 in 1 design, there is a lightweight, middleweight, and heavyweight, so you can change based on %s. Plus theres that invulnerablity frame from what I gather from the screenshots. This is really shaping up to be quite the character.
I figure Squirtles Up+B to be Aqua Jet, Ivysaur's a variation on Vinewhip, and Charizard just Flys.
And some weird things, Pokemon calling other Pokemon out of Pokeballs, and maybe a cool move, the ability to catch other fighters, if they don't mash the buttons fast enough they are caught and game over!