Three average built people can sit in the back of a European car, but in most cars it’s not super comfortable. Car seats tend to be more solid than an average built person (humans are squishier) and are bulky in strange places. Plus truthfully, there’s a 0% chance I’m trusting a lap only belt to hold my baby in the middle seat in a crash, because that seems like a real fun way to turn a baby in a car seat into a projectile. I’m not saying I think people should demand strangers pay for a bigger car, but I don’t think it’s outlandish that someone who drives a small hatchback (which is very normal in Europe and they can be very tight on space) might need to think about a bigger car when they have 3 car seat aged kids.