Hi sorry I forgot about this thread and have just come across it again!
I would try to keep your 3yo in a harness for longer if you possibly can. 3.5 is quite little to move to high backed booster and ideally they ought to be as close as possible to 18kg and 105cm when you move them as the safety protection of a high back booster at 15kg/100cm (the minimum legal requirement) is not as good.
It's likely that your 6yo's seat is allowed to be used without the isofix - do check this. If it's just a high backed booster then these tend to be a bit slimmer than the combination harness + booster seats. If it's a combo one, consider giving this to the 3yo in harness mode, and moving the 6yo to a slimline high backed booster. Maxi Cosi tend to be slimmer and Britax have Adventure Plus2 or Kidfix i-size which are fairly slim. Other than this, I'm out of date so best thing to do is literally go to a car seat store (try looking on the Britax website for stockists, and see if you can find any which aren't a chain) and try different models out.
If 6yo's seat is already just a HBB, you could look at trying different harness+booster combos for your 3yo - unfortunately these tend to be dependent on the isofix and not especially slim. But you could look at Britax Evolvafix, Cozy n Safe Hudson, Joie Elevate R129, Maxi Cosi Titan i-size.
Longer term it might be beneficial to look at a Swedish type ERF seat for your new baby when they grow out of the infant seat, simply because they tend to be excellent for fitting 3 across due to the combo of tethers and seatbelt making fitting flexible and the bases are slim and the wide part (the shoulders) is positioned away from other seats. Look at Avionaut Sky, Axkid Move, Axkid Minikid 4. If your 3yo is amenable to rear facing, you could even get one of these now to replace the Britax seat and then later when the baby needs to move out of the infant carrier, the 3yo might be old enough to go for a high backed booster (or you could try the Britax, or you could look at a cheap-but-smaller seat like Graco Extend.)
As said, infant seats are much of a muchness size wise. Maxi Cosi, Joie, Britax, Cybex, Avionaut all have the buggy compatability - you may want to look at factors such as whether they have stylised bits on the carry handle which stick out annoyingly and interact with the other seats.
Some Joie infant seats have the option of a belt fitted base, if that is useful - sometimes you can fit these in the centre, which gives you more options for the others and saves leaning over a seat as you just click the baby seat on and off the base. However, a lot of cars do not allow a support foot leg in the centre seat, which can cause an issue here.
Cars with 3 isofix points in the back row (or middle row if it's a 7-seater) do exist and tend to be the ones with 3 full-sized seats, but they are more expensive. It's possible to fit three car seats across in an ordinary estate car with a 3/4 size middle seat, but you have to be willing to play car seat tetris in a car park of a baby shop somewhere, and it means you'll have that issue every time somebody moves up a size, and might be limited in what arrangement you can have the seats which can be a pain if you get any behaviour issues between two specific siblings.
Overall widths of car seats as given on online shopping pages are not very useful to you, you are best off physically trying as many combinations as you possibly can because in general, it is the shape of the seats and whether or not it's possible to fit them together and still fasten everyone in place. Belt extenders are not safe and anything that changes the isofix is not safe. Also watch out for buckle crunch (where the buckle lies across a hard part of the car seat shell) if you are fitting multiple belt fitted seats.
You can get away with a less well-rated high backed booster in the middle seat or even a backless booster perhaps because the side impact protection is not as crucial in the middle seat. However you should check that the lap belt has a good placement (across child's pelvis or top of legs touching pelvis) and that the shoulder belt doesn't rub their neck.
You can also safely put a rear facing child in the front as long as you can disable the airbag. Forward facing in the front is more risky, because the airbag can still be in the wrong position for the child and they do inflate with a good amount of force. Check your car manual for advice. Some cars (e.g. Peugeot 3008) have isofix points in the front passenger seat.