I thought of you this weekend when the Sunday paper arrived. I notice COSCO brand car seats are quite narrow when that silly accessory cup holder is removed. I saw it in the Wal-mart ad.If you had 3 kids in that type of seat you likely would not have to remove the rear arm rests.
I have seen 4Runners with both of the rear seat arm rests removed in order to fit 3 baby seats across that back seat. Booster seats for slightly older children tend to be flatter-thus wider.