[mko9]

Short answer: Yes, there is enough space. But I don't have an exact measurement. The Diono Radian mentioned below is 17" across.

Long answer: I currently have 3x car seats across the back seat - 2x Diono Radian (forward facing, middle and left) and booster w/back (right). We have never had it configured as you are talking about, though. My 5'5" 120lbs wife or her 5'5" 110 mother both would fit in the middle. They frequently do this in our Land Rover LR4, which most times also has 3x car seats across the back.