This depends on the combination, types and exact size of child seats you plan to use. At the very least, the car will need three individual rear seats, each with a three-point seatbelt, but the only way to be sure is to try the seats in the car.
We wouldnt advise trying to squeeze three seats across the back of a regular five-seat car because its unlikely that all three seats will fit properly. Instead, put one in the front passenger seat an adult passenger may be able to sit between the two rear child seats.
An alternative is the three- or four-seat solution from Multimac, which allows you to fit up to four children into one bench-style seat across the back. Prices start at 1099 log on to www.multimac.co.uk for details.