There is a company called Sunshine Kids that sells a narrow car seat. I was able to get 3 across in my M5 using two boosters and the Radian. The Radian is expensive, rattles a bit and is heavy (21 lbs). It does work well if you need to get 3 kids in a small space. Be aware that the third row in the X5 does not have Latch anchors, and two of my kids (ages 10 and 6) hate riding back there.
Sunshine Kids Car Seats :: Sunshine Kids Juvenile Products