Our back seat is full with 3 kids. We always had the first one on the passenger side & not in the middle.
Now we have DS4 on the passenger side as he can do up his own belt so sometimes it takes longer, DS8 sits in the middle with a harness belt on (when not in frount), DS3yrs sits behind the driver.
I can not have the 2 little ones together as they will fight when you are about 5mins in to your drive
But want to put DS3 in the middle & the other 2 on the sides, as there will be more room.