Do you recommend a group 1,2,3 car seat or just a group 1 car seat?

When my 1 year old outgrew his group 0 seat I shifted him to my 2.5 year old's group 1 seat, and got a group 1-2 seat for my oldest. He's tall for his age and wouldn't have stayed in the group 1 seat for long, so it was a reasonable compromise.

I wouldn't put the 15 month old in that group 1-2 seat, it clearly offers less protection mostly for side impacts. A 15 month old's head is just much heavier compared to the rest of the body than a 2.5 year old. So even the group 1-2 seat does make concessions on safety to make it fit kids of a wider age range than just a group 1 seat. Those concessions would be bigger for a group 1-2-3 seat.

I know the group 1 and the group 1-2-3 seats are marketed for age 9 months and up, but I really wouldn't recommend them for a 9 month old unless it's one of the newer models that still rear face for that age. And preferably until at least 2 years.

The EU countries are really behind the States in that respect, there is new legislation coming for extended rear facing, but most available group 1 and group 1-2-(3) seats still only forward face. If your child hasn't already outgrown the group 0 seat yet (in weight and or height, especially whether the head comes up above the seat) I would keep it in that seat as long as possible. More brands will be coming out with convertible group 1 seats in the near future because of the new legislation. That means you can rear face up to 2 years, and then turn the same seat around for forward facing. Much safer that way.

In the USA we have infant seats (rear face only) and convertible seats (both rear face and forward face).

My preference was to get one car seat - a convertible. We used the DIono Radian seat for our 5 pound newborn. He is now age 5 and still can use the harness. Yes the seat was HUGE. The hospital recommended a loaner "infant seat" because he was so small. When they did the car seat check, the lowest infant seat strap slots were above his shoulders - a no no. The HUGE convertible seat was a perfect fit for him. So home he went in that big seat. We love the seat. We rear faced to age three in that seat. He now is forward and fits fine. We put his tall 7 year old sister in the seat. She barely fits. LOL the seat is labeled "to 100 pounds with seat belt or 65 pounds in harness". But no way my kids will ever be that big by age 8.

http://us.diono.com/convertible-to-booster/radianr...

This is what we have, we had to use their infant insert to fit the newborn snug.

Out of the car or on shopping trips I used a ring sling carrier or Ergo to "baby wear". No stroller or carrying some heavy baby car seat.