Not all car middle seats are the same. Just because there’s a seatbelt there, doesn’t mean you can get a good safe fit for a restraint, or that 3 seats may not fit across.
Check the width and hump of the middle seat and the total USABLE width of the row to make sure you can future proof your vehicle purchase. The @babythingszetland team are happy to answer questions about different vehicles. Remember that restraints all have different widths too.