narrow FF 3 across FF harness/booster option

We are planning what carseat/booster purchase we need to put 3 across in a Volvo V60. The car will have built-in boosters on the two outer seats but probably only one child can sit height-wise in one at first.

- 9 year old - tall but doesn't 5-step yet

- 5 year old - 40 lbs and short for his age - will get his seated torso height tomorrow

- infant - currently 6 months - will ERF

Our current available seats are:
- Recaro Vivo HBB
- TrueFit Premier - too wide
- Britax FixWay (9 years old so encroaching on the 10 year guideline for a Swedish seat - will need to be retired soon)
- Britax MultiTech (purchased in Sweden 5 years ago - approx. 5 years of use left)

Long-term, we'd like the baby in the MultiTech on the side or a new narrow seat on the side or middle. 9 year old will be in a built-in booster. 5 year old in ???? 5 year old currently rides RF in a MT most of the time but occasionally rides in the Vivo. He sits better in a booster than our 9 year old and we would be okay with him in a booster full time come summer. I want him to be able to buckle himself though and the Vivo basically sits on top of the buckle when we tested it so are wondering if there is either a much narrower HB booster or a narrow harnessed seat that will last him until he is tall enough to ride in the built-in booster. The harnessed seat could then be passed to the baby. Alternately, if we put baby in the center, what narrow seat would you recommend? Radian? Foonf? something else?

FYI, we are in the US right now but FixWay and MultiTech were purchased when we were in Europe. The new seat scenario will be used in Europe at first for a few months and then the car will be imported to the US. We plan to purchase a new seat for this car in the US before we go to Europe. Thanks for any narrow booster or harnessed seat recommendations!

Any thoughts for us on narrowest harnessed seat or booster to make this situation work so that the 9 and 5 year olds can buckle themselves? The belt was under the Vivo when we tried so that is too wide. We've heard that the Volvo boosters (not the built in ones) are narrower in the back than in the front allowing hands to get down in there to buckle.

Thanks for any opinions!

There are a few backless booster options shaped like that- Harmony makes a few. I'm not sure how they'll fit an older kid but it's worth a try.

You could look at the Harmony Defender for the five year old, if you can find one in stock, or an Evenflo SecureKid (not quite as narrow but sometimes puzzles better.)