I'd at least try it in the middle to see if it really doesn't fit, that's where we keep our chicco keyfit 30 and it has plenty of room to clear the seats in the X5. I don't remember the X3 being that much tighter in width that it would cause an issue, but you may just have a huge carseat.
17 Volvo XC90 T6 Inscription P* (OSD 7/16)
15 BMW 328d xDrive Sports Wagon M Sport
15 Audi SQ5 Prestige
13 Honda Insight EX