The AU that I have now does fit 3 x seats... (i've got a 5yr old, a 3 yr old, and a 1yr old) so they are in 3 x different carseats...
though - it is mighty tight... luckily the older one can fit his hands between the seats for the seatbelt buckles...
That would be my biggest concern with a large car, is that I can actually close the doors with the 3 x seats !
maybe I should take the seats with me to a dealer!! they'd love that...