I have an 8 yr old, 7 yr old and 2 yr old- all girls.
For trips of any distance, the girls must be separated or there is no peace in the car.
With the carseat and the older girls all on one row, it's a tight fit. Plus you lose any benefit of armrest and cupholders.
Asking my older girls to sit in that "third row" of the X5 was not a good proposition- not on long trips. And the wife insisted that the carseat be accessible so it has to go on the 2nd row.