Well I've just tried to fit three car seats and they don't fit.
I bought two (allegedly) narrow group 0+ infant seats and tried fitting with our existing narrow group 1 seat (which we use for aircraft). They simply don't fit together. The infant seats sit at an angle and they actually touch the doors. A one-off short journey yes, but as an ongoing concern the X3 will not fit three car seats.
I'm going to look at Multimac, but this is very expensive and will reduce the flexibility of the car as it is a semi-permanent bench across the back seats.