"BubbleBum, the cost-effective, portable, lightweight, brightly coloured, inflatable car booster seat for children aged between 4-11 easily deflates and folds flat so that kids or mums can carry it in rucksacks or handbags, making it perfect for holidays, rental cars, taxis, coach journeys, school trips or car pooling" it costs around £29, and is legal/safe.
It's okay... it would definitely fit between almost any two carseats/booster seats, it's ideal as a cheap solution for short journeys like the school run, but we tried it on its own (we don't have three kids yet) for a 3 hour long journey, and it wasn't so great, ds fell asleep and was just flopping everywhere without the support that his proper high backed car seat provides. It does have a strap which is meant to keep the seatbelt from sawing their neck, didn't work fantastically well but you can get one from mothercare for about a tenner.
I have a pampero seat form Halfords which is very narrow. Has a 5 point harness which can then be removed to use the seat as high backed booster with seatbelt. We've managed 3 in the back with it. One in that seat, one in another high backed booster that is slightly wider and the baby rear facing. You will need to play around with order of seat to make sure they won't push on seat buckles.
We have three in the back - the high back booster for my 5yo is this one which has no side wings at hip level My DSs are both in a 5pt harness seat that has a very small footprint, and sits quite high so doesn't interfere with the seat buckles for the high back booster. i'm afraid I don't know what its called and can't find a picture on line! HTH!