Nearly every sedan out there these days can do 3 across. You just need narrower seats with smaller vehicles.
I think the carseat issue is width and length.
Width: the Diono Radian wins, as far as I can tell, on width. And its great for travel, but heavy.
But. It's hard to use rear facing in a small car. Or to be more specific, it would be hard to have two rear facing ones in a small car. We were able to do it, but only in my husband's car in the passenger side. He would not be able to use it on the driver's side, because the seat was too tall, and he is 6' (but I'm 5'2").
Height: Britax Roundabout and marathon worked pretty well for us. They are easy to install, and not as tall, so you can use 2 of them RF in our cars (Civic, Matrix).
However, width, you cannot do 3 across.
Luckily, our kids are 6 years apart. So Kid #2 was rear facing until a little over age 2. We switched him because he kept kicking his big brother in the face - we had to have the seat in the middle. Kid #1 was in a booster.
Now they are 10 and almost 4, so we have a 5-pt booster for the little guy and the big kid is out of carseats.
I have friends who have 3 kids, all in carseats for a total of 4 years. There was no way to do that in a car with only one backseat. Third row was necessary.