It's definitely possible to put 3 car seats across in the back seat of pretty much any car available in the American market. The appropriate narrow seats just aren't very popular or well known...
So we went for (especially in Europe) rather limited subset of cars where all 3 of the 2nd row seats are proper sized, with Isofix on each of them.
Usually same makes/models that offer the option of additional 2 seats in the 3rd row.
Every family I see with at least 2 kids has a minivan, so maybe we can discuss if minivans are causal.