We are just going through the very same thing- I had DH panicked for a while thinking we were going to have to upgrade our car. We have a Mazda Tribute SUV and I was able to fit 3 seats across the other day, DD is almost 4 and weighs about 19 kgs, so she is in a Vario Booster with a protector harness, this was rated the top booster seat and i *think* has a 5 star safety rating. DS is in a mothers choice seat, which I am hoping will do him untill he is heavy enough to go straight to a booster, because I am yet to find a slimline convertable 8-26kgs convertible booster/ car seat.
Then I had our SNS compaq in as well in the rear facing position. We are going to actualley use the a capsule for the first 6 months or so, and then bubs will go into the compaq.
I did find I had to move the other 2 car seats out towards the door and I am going to have to use towls or something to make sure they sit flat due to seat shape, but there was def enough room for all 3 and it wasnt like they were busting out of the doors when we opened them.

Good luck finding a combo that works. If you have a look here it gives you a few different brands of car seats and their measurement, which might help.