How to arrange 3 carseats?

We have a Ford Expedition, and currently have our two children (ages 2 and 3) in forward facing carseats-each positioned behind the drivers and passengers side seats. We are expecting our third child, and have installed the rear facing infant seat in the middle between the two boy's seats. It seems like too far back to send one kid to the second set of back seats. all 3 carseats fit comfortably and arent even touching, but is there any sort of safety issue with putting the rear facing seat in the middle instead of by one of the back doors? both the boys can climb into the vehicle themselves and get in their seats and wait to be buckled in, and the infant seat lifts easily between the front seats to take her in and out of the vehicle without having to carry it over the other carseats.