We have the Babylove Ezi Combo Seat for DD and have put on layby 2 more for the twins. They do from 8kg's to 26kg's in weight, it has the 5 point harness for when they are smaller and then when you need it as a booster seat you can either use a harness or the normal lap/sash seat belt. They are quite inexpensive (we paid $190 each) and are not too wide so we can fit all 3 seats across the back seat of the station wagon.

We decided this was the best option for all 3 kids until they are 7, and until the twins are big enough to go into the seats we are going to hire 2 capsules.