You probably need an MPV - three individual seats across the back is hard, in fact basically impossible, to find in anything else. A Volkswagen Touran, Toyota Verso or Kia Carens are your best bets. 

 

However, if you don't fancy an MPV, there is another option. You can buy kits for the back seat that allow you to carry three to four child car seats in a row. They cost around €1,500 and will fit most saloons, estates and SUVs. If that's the route you want to go, then I'd suggest an estate (three kids = lots of boot space needed) and so I think you should look at a Mazda6 Estate, a Skoda Superb Combi or a Hyundai i40 Tourer.