We drive a Ford Mondeo with three kids in the back. Pretty sure a Mazda 6 would be ok too.
The only problem is that most of these cars don't have 3 anchor points, which is ok for us now as DS1 is in a half booster.
I am looking into a seven seater like the new Nissan Dualis or the Captiva as they don't seem as big as the other SUV's on the road.
Another to consider is a station wagon as you can fit three in the back and put one of those d i c k i e seats in the back as well if you are picking up kids and their friends.