On her honda odyssey I can fit 3 in the 3rd row and two in the captain chairs in second row, on the expedition i cant fit 3 in the 3rd... only two, I can fit all 3 in the second row but it's far more annoying that way cause the second row seats only slide forward so much to allow access to third row.
Oh and the trick to it is you place all three car seats down on the seat then while they're in position you start strapping them down, if you strap two car seats down you wont have any play and adjustability to force fit a 3rd car seat so you have to keep them loose and keep them in position while strapping... its frustrating and aggravating and requires the smallest hands to fit between the car seats to attach the clips in and buckle the seat belts.
But to also answer your question I have a 5, 3.5, 2.5, 1 year olds and the 5th one is an infant 2 months old.
Does a 5yo in a 3rd row seat even warrant a child seat?
Cant see how the heck that is safer. Front seat and airbag nonsense, maybe, but what exactly does the child seat add in a 3rd row seat for a non-infant/non-toddler?