We did have all ours on the back seat (the middle row) because all three car seats fit on there. Baby behind passenger, oldest in the middle, middle child on the outside behind driver.

Now we have:
Baby behind passenger, oldest and middle child in the far back row. We have taken out a seat behind driver so the kids can climb in behind the passenger seat (from the safe side) and then get into their seats, and I can climb in behind the driver's seat (need a lot of room for my big preggy tummy and bum!) and secure them in their seats.

When the new baby comes, new baby will go behind passenger, and third child will move into the middle of that row, and I'll still have room to get in and out and secure the other two into their seats.

Phew. Confusing.