No, that will not work, not for comfort anyway. You can fit a child in the middle using the centre seatbelt, there are no isofix points for the middle. But when doing so you will find that the car seat overlaps the two outer seats. The middle seat is a fair bit narrower than car seats. For a short trip it might be bearable - just. Doing it the other way, car seat on left or right will be just as tight for the poor person in the middle. I had the exact same issue with my C class previously and the XE is bit tighter in width if anything. Even with just two car seats it is a struggle as the car seats are a little wider that the seats and partially cover the seatbelt slot. I think this is more of a reflection of the chunky design of kids seats than a problem with the XE.