What's the age range/height specification on your 3rd row seats? Mine is 4-12 year old. dd2 had a big foam booster, but the seat belt didn't fit around it. I bought her a half seat booster, but she still sat a little high in it.
She was 5 when we bought the car. We stopped using the booster in the 3rd row, because I paid attention to the age limits. The seats are smaller. They are booster seat size. In our car the 3rd row is too small for a car seat. It actually makes it more dangerous IMO. But I don't know the details of your car.