Difficult, but not impossible. We fit 3 across the back seat of our Mitubishi Lancer for a few years. You do need to be selective in which car seats you get, and I wish it was easier.
In my country we have had in the last decades the following - car seats, prohibition small children to roam free, prohibition for kids under 12 to be left alone at home, the size of the city apartments has shrunk substantially - from two bedroom to one. The impossibility of stay at home parent. You can add to the list.
https://www.amazon.com/Graco-SlimFit3-Forward-Highback-Kunni...