Hi - My friend in London who has 3 littlies has a MULTIMAC in her Prius which means she can take her 3 + a friend's 1 in the back. It's really a wonderful invention. Quite expensive but much much less than a new MPV or whatever.
It's very very safe cos anchors to the seat belt mountings so much safer than a reg seat. You can get a rear facing baby cradle too. Really it's solving so many problems for them and I should think for loads of families who can't get 3 in the back of a regular car.
Also she has a mate who has a 3-seater in the back of her Focus! [and it will fit a mini and little Fiat too!]. Why isn't there more publicity about it. I think only way of finding out more info. is www.multimac.co.uk.