Thanks! The Bubble bum wouldn't work for us (and they are changing recommendations for the safety of those apparently...) but that does help with a sense of the width potential!
It may be possible to find a medium set that might go three across if you needed to always have them in place. I just needed it for a one-off so used what I had handy. Specifically the two I had on the sides were these:
AFFIX? Youth Booster Car Seat with Latch System | gracobaby.com
2018 Pacifica Limited (gas) - Oct. 2017 build