I have an 535I with my 6 year old in the back right with a booster seat, my 11 month old in the middle and my wife in the back left no problem. Wife says she has a plenty of room and she is not a 110 lbs Lady
soo. But we don't use the base for the rear facing baby seat it has built in spots for the seat belt to secure the car seat. Saves a lot of room! We downgraded from a 7 seat 2011 Durango RT to the 2013 535I M Sport. Fantastic car!! It's no problem for us but you might strain your back getting the middle child in and out of the F10. By the way test drive a X5 much better getting the middle baby in and out.
Good Luck!
James