Lol, I asked this a couple of weeks ago.
We have looked around at cars and have (we think) decided on a Xsara Picasso. They are kind of inbetween an mpv and a car and they have three seperate seats at the back so you can easily fit in three car seats/boosters/etc. They are also quite reasonably priced.
I also find this Which report very helpful, they tested 15 cars to see which worked best:
http://www.which.co.uk/baby-and-child/baby-transport/reviews-ns/three-child-car-seats-in-15-cars/
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