Hi M.. We have an Acura MDX as well and I have three kids (7, almost 5 and almost 1), so I figured I'd respond. I had the older two in the third row for a while, but then decided to put them all in the middle row (baby on end, with two booster seats next to him) as I found it was easier for me. The way we do it is that I buckle the baby in, whomever is sitting middle middle buckles in by moving the window carseat all the way over to the door, then the person by the window either is helped by me buckling them in (I slide the carseat all the way over to the door, buckle them in and then slide the carseat back over so the two carseats are touching) or if it's my oldest child, she can buckle in and then shimmy her seat over by herself. Our booster seats are Graco and our baby is in the Britax (Marathon I believe).
Hope this helps you out. Your 5 year old should be able to buckle in shortly (if he can't already), which will make your like easier. It's amazing how the little things make such a difference!! Good luck!