It'll be hard to say. Are you using a convertable seat for the rear facing or one of those baby carrier systems (like the Eddie Bauer/Safety 21 that I have for my kid)?
Honestly, in terms of size of the back seat, your best bet would probably be to go down to the dealer and try fitting all 3 seats side by side. When I was shopping for a Pilot to replace my wife's Civic, the dealer was more than willing to accommodate me in test fitting the seat in the car.
So, I really can't say on 3 seats (since I only have 1 kid), but if anything, I would probably put the 2 bigger kids in the forward facing seats on the outside and the rearward seat in the middle (or how I've dubbed the "bitch" seat).