This is an actual conversation I had to have with my wife, having a third kid definitely requires a larger vehicle as the vast majority of cars today simply don't work for a family of five. Child car seats are too wide and the backseats of most cars are too narrow, and the price jump to a vehicle that would be able to handle three child car seats at once is enormous. Throw in some dogs, a couple pieces of luggage tha…
Even if you find a seat configuration that fits, the next hassle that drove me nuts was getting access to seatbelts easily. Taking the kids anywhere is that much more annoying when you can't rely on the oldest at least sorting out their own buckles. (Only just occurred to me that a seatbelt extender would probably help bring the receiving part of the latch up from the depths between the seats -- too late now, bought a different car.)
Almost worth staggering births so that after three years the eldest is able to handle buckling themselves and then siblings. Worth finding every advantage like this when you're outnumbered by children.