If your kids have friends, you'll probably find yourself driving around as many kids as that fairly often. I was raised in a family of five and my parents had a minivan because nothing smaller would have been practical. Even if you can technically squeeze 3 kids into the back of a sedan, good luck going more than 30 minutes away without fights breaking out. And even if your kids are particularly well mannered, they still won't be comfortable. If you're driving a few hours to visit family, do you want to have a headache from fighting kids when you get there? Or kids in sour moods because their sibling wouldn't stop elbowing them?

With today's car seats, it can be made to work but it's a challenge. Realistically 2 kids in car seats makes the middle seat of anything but the largest cars impractical. At which point you are going up an extra row of seats, 6 seat layouts are uncommon.

In practice, Minivans seat 7 - Two adults, three of your own kids, and two of the neighbors.

You can't fill every back seat with a car seat. You will have issues accessing the rear if you put 3 in the middle row, the rear seat often isn't wide enough to accommodate three seats