D.K.
They are not allowed to sit in their booster seats. All carseats have a sticker on them somewhere that says if they meet FAA guidelines. You can gatecheck the carseat if you do not luck out and get a seat for the baby. However, this does cause damage to the carseat even if it is not visible and some people strongly recommend against checking them at all. I would take the carseat on and sit your three year old in it. Isn't 3 kind of young for a booster anyway. In PA they are still required to be in a carseat until they are 4 and it is strongly recommended until they outgrow them (65 lbs for the bigger ones).