The radian is the thinnest. That's right. It costs a pretty penny too. About $275+tax here in Ottawa. I would recommend checking out the forum at
http://www.car-seat.org/. A lot of people there have had major success in installing 3 car seats in the back. That's where I went to get inspiration before I fit 3 in the back of my teeny-tiny honda civic. And I didn't even have to get the radian in the end. You can use the search feature to look for threads that talk about 3 seats across success stories. Some of the stories there talk about which car they had and what seat types they used to make it work. Good luck! :)