Fortunately for me not a problem that I've got at the moment, at 33,29 and 25 mine have their own cars and even give their old man a lift occasionally. Back when I was juggling kids in a car I used
Incarsafetycentre in Milton Keynes, they've been around that long and are still going as far as I know, they have been specialising in child seats for years and know their stuff. Being in the Cotswolds you're not a million miles from MK but as you don't have the car yet I'd give them a call and see what advice they can give. I vaguely recall a similar thread to this a few years back and with modern cars and bulkier modern child seats getting three in the back is an issue but there was a system which was specially made to accommodate three children with isofix.