Combination seats are those are FF and turn into a booster. The ones we recommend most are the
Evenflo Maestro (around $80, FFs to 50lbs, boosters to 100lbs),
Graco Nautilus (around $140, FFs to 65lbs, boosters to 100lbs), and
Britax Frontier 85 (around $200, FFs to 85lbs, boosters 120lbs). If you buy three combination seats instead of three more convertibles, you'll be prepared for the next stage in car seats when your DS is ready.