[mudetroit]

Speaking as an owner of a small car (Honda Civic), and 3 children in need of car seats (a Slimfit Graco which is a bit wider then the one you listed, but 2 of them are now in boosters which are narrower to begin with.) The problem isn't always pure width.

The rear seats in are contoured, for comfort reasons, in such a way that you need to put things under one side of the seat to level them out properly. And then the seats are so close together that leaving access to the seatbelt sockets for the children in boosters to access. So simplistic looks like this give you a really inaccurate picture of the problem.

I will also go further and say that even in larger vehicles getting 3 seats across any single row is hard. Adding a 3rd row doesn't even necessarily solve all of the problems if there isn't an easy path out if you have two kids needing 5 point harnesses done by the parent. Those seats block the folding exit mechanism that a lot of 3rd row vehicles use to let passengers in the rear row exit. You would have to remove a seat entirely to let that action work.