I think rayxi is correct, you could fit three in, put it would be a lot easier if you pull the side bolsters.
I have two cars seats in the back of mine, rear facing one in the middle for baby and forward mount seat (with five-point racing harness ) for toddler.
If you have more than one rear-facing baby seat, you'd probably have some issues, the passenger seat has to be scooted way forward to fit a rear-facing seat directly behind it.
Wife absolutely LOVES having the X5 to drive around in. I hardly ever get to drive it.

And PLEASE don't buy a minivan!!