IMO (there is a lot of opinion on this 3 seats will/won't fit thing rather than hard fact) you can get these comfortably in a focus - I had a Britax Babysafe infant carrier, a kiddy infinity pro (stage 1 9m to 4yr) and a britax adventure (15-36kg) in a 2006 3 door focus. Also a cybex hum... forgotten the name. High back booster. Bother. Anyway, it was a big one, the kiddy seat is fairly wide but at the top not the base. Focus was comfortable, doing up the belts was fairly easy.
THEN when that got close to death from over use we got a 2011 Seat Ibiza (yes, smaller, supermini class car) currently have the britax, the kiddy infinity and a graco high back booster in (as this was the narrowest I could find at 41cm, the britax adventure is 44 but does actually fit too). It is do able but to be frank a massive squeeze and doing up the belts does involve swearing at least half the time. However I had them checked by a car seat retailer I trust (independent one) who said there are no problems with the installation. He said even in huge cars and people carriers the seats almost always touch.
So if they fit in a teeny ibiza I'd be willing to bet they fit in a lot of mid size cars IF you have the right combination of seats. Also you say booster, if you mean a regular booster then you should have no problem at all finding something as they are narrower than high back ones usually.
Multipla, Picasso, meriva, those sorts of cars with 3 full adult seats in the back. DD's were 2 & 5 when DS was born and while 3 seats fitted in our Fiat brava, we had to put the armrests of one under the other etc to make them fit. Got hard work very quickly! We got a multipla & it was a revelation to be able to put all the seats in & still have room between them to put your hand in & buckle them up!