You'll have to play car seat tetris - identify a few models you want to look at and check the manufacturers' websites and then phone around local stockists to see what they have in stock and whether they'll let you have a play. Small independents may be more likely than bigger chains. They may also be good at suggesting other options. Sometimes the slimmest spec isn't necessarily the whole story because it's about how the seat bases are shaped and how they fit together. For example we can fit our baby seat in the centre but only when it's on a base, because when it's not on the base the handle is too low and overlaps the toddler seat next to it. (Luckily our base is belted!)
Once you've got a few car seats to play with it's simply a case of trying them all out in different positions and seeing whether any combination fits. It's worth measuring your side rear seats - they may not be the same width. You want to avoid isofix if possible because it positions the seat rigidly in one spot. Belt fitting gives you a little more flexibility. Just beware that the eldest (once they are in a booster) doesn't accidentally undo another child's seatbelt holding their car seat in by mistake - you might want to put them on the side where their buckle isn't next to the centre one. You may want to consider a belt buckle guard (you can buy these on amazon). You may have the same issue if you end up belt fitting the baby seat and taking this in and out daily.
Evolva is pretty wide. You might get away with 1x Evolva. But it may be worth swapping (one or both if you don't want to RF) for a slimmer Group 1 forward facing seat, which probably means one that doesn't convert to a booster. Britax Eclipse is OK, it is definitely slimline, but quite old fashioned now and doesn't have a lot in the way of safety features. I'd look at Maxi Cosi Axiss (not Axissfix) (you may not be able to use the swivel), Maxi Cosi Tobi or Britax King. These are all slimmer options, belt fitted, good safety test results and have some kind of tensioner which helps with fitting securely. The problem with a lot of cheaper FF seats is they have nothing holding the belt tight. The Evolva has the serpentine belt path which works well but this tends to be unique to 123 seats, and I don't trust any of the slimline 123 seats.
Once the 6yo meets the capacity for a booster you can swap them into a slim HBB - Britax Adventure or Maxi Cosi Rodi (Rodi air protect has better head protection and is the same size) are slim ones. How far off 15kg are they? Or did you mean they are too little for a backless booster? These aren't typically any slimmer at the base than a HBB without horns anyway. I agree you shouldn't put him in the middle with no seat yet. It might be legal but it wouldn't be safe for him if you crashed.
Try the Maxi Cosi without the base as well as with the base. If you have the compatible seat, consider changing to the Easyfix base which can be belt fitted. Again it just gives you a few more millimetres to work with. I don't personally think it's worth swapping to Joie Juva because you don't gain that much in width.
A wild card option - you may consider moving the 6yo or 3yo to a rear facing 25kg seat. Not because I'm an ERF nut (I'm not really although admittedly it is safer) but because the 25kg RF seats have so much flexibility in fit and can be fitted on the edge of the seat vs right back into the seat so you have a lot more options for space to play with. Plus the top part of the seats are the widest and they're crucially not in contact with the top of the high backed booster etc. Axkid Move/Minikid and Britax Max Way are the slimmest ones. If your 6yo is tiny, you could actually stick them both in one of these but they are quite pricey, so it might make sense just to get one.
Joie have a couple of slimline Group 1 seats - Tilt / Steadi, but I wouldn't personally recommend using them FF, only RF. So might be an option for the 3yo if they are willing to rear face, possibly as a temporary option until one of the other seats is outgrown by a sibling. They are cheaper so suitable as a temporary/spare seat. The baby could move into it directly after the baby seat too.
If you can disable the airbag, consider putting one child in the front seat, particularly if they are rear facing.