According to Serebii, it has been confirmed that Fennekin, Chespin and Froakie are pure Fire, Grass and Water-types.

That would be fine with me if they're all pure fire, water, grass. We've had each already at least once. Blastoise (water), Sceptile (grass), Typhlosion (fire) and many more. Those are all pretty neat pokemon, and I'm sure it'll be pretty neat when the secondary and final evolutions come out too! I can't wait to see what they turn out to be.

That doesn't remove the chance of them being double type. I am confident that at least the fire starter will gain another type, and I do hope all three of them get another type, since single types these days can't compete much.

I think the starters becoming duel types that form anouther type triangle would be pretty cool and interesting, have it go the other way around so grass isn't weak to the fire type and what not.