It depends which child seats. Some are ridiculously wide and three will not fit across the back of anything. But if they are a sensible size your answer is probably a Ford S-Max 2.0TDCI. But you won't get much of one for £8k. Will have already done 100k miles whether it says so on the speedo or not.

Smaller, maybe an old shape Citroen C4 Picasso if you can stand to look at the thing. More robust with the 2.0HDI engine and manual transmission. The other way is to buy a three in one child seat. Quite expensive, but not as expensive as switching cars: multimac.co.uk/3_child_car_seat