Gen I: Grass/Poison, Fire/Flying, Water
Gen II: Grass, Fire, Water
Gen III: Grass, Fire/Fighting, Water/Ground
Gen IV: Grass/Ground, Fire/Fighting, Water/Steel
Gen V: Grass, Fire/Fighting, Water
I think it's a longshot that all 3 of them get alternate typings considering it's only happened once. Of the 3, based on their first evo, Froakie looks, to me, the least likely to pick up a secondary typing. I think the Fire/Psychic case for Fennekin is a strong one and would frankly be awesome.
I'm not sure what Chespin might becomes because, of the 3 starters, he has the most opportunity for change over his evolutions. I mean I really could see him becoming so many different things.
Froakie certainly could pick up an alternate typing (Ice as others have mentioned seems perfectly plausible), but he looks pretty run-of-the-mill Water to me, in his current form.
One final note is, even though I think Fennekin is the strongest lock for an secondary typing, the Fire starters have been gifted with secondary typings most. Only Typhlosion remained pure Fire.