Iíve done three boosters in the back but the one in the middle I had to use a much smaller one (bubble bum). Outside ones were standard graco boosters. Little effort making sure everyoneís clipped but all worked out and felt like all were safe and secure.
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!