It totally depends on the car and the ages of the children.
The problem is you can't always rely on simply "slimmest", often what is best to do is look at the shape of the different seats as they often have sticky out bits, and find a way to tesselate these best.
Another tip is to take a tape measure and compare both rear side seats - often one is 2-3cm wider than the other, they aren't exactly the same size.
Best thing to do is find an independent retailer who have knowledgeable staff who are happy to stay with you and try umpteen seat combinations in the car until you find the magic combination that does fit. That will get you the best overall result, I think.
In order to do this I'd look up a fairly common car seat brand such as Britax or Maxi Cosi and use their stockist finder to locate stores near you, ignore all of the chains and look for names that seem like they might be independent stores, then contact them directly and ask whether they'd offer this service.
It's a shame you can't rear face in the middle as you can often get the best fit by alternating direction - Forward / Rear / Forward. However you could get a rear facing seat for the 3yo and tesselate them that way - Rear / forward / rear.
In case you decide/need to do it alone, I'd first look at Britax Adventure or Maxi Cosi Rodi as a slim booster for the 6yo. If you have a higher budget and want something more premium, Besafe izi Flex Fix is a good one. The isofix is optional. Which booster do you have at the moment? If it's not super wide it might be worth trying to work with what you have.
Every Stage OTOH is huge (4-8cm wider than most other seats) so I would probably discount that altogether. If you are using forward facing with the harness, you may also want to know that it didn't test very well that way and Which? recommend not using it.
Do you have an infant carrier? These tend to be quite slim anyway but if you need to purchase a new one, I'd focus on getting one which is easy to belt in - probably a Maxi Cosi. Seatbelt fitting gains you a bit of flexibility and space.
If you did want to look at rear facing the 3yo you could look at Britax Max Way / Max Way plus and Axkid Move / Minikid. All 44cm wide. Max Way Plus and Move are both on offer at the moment so cost less than usual. These last until approx 6-7 years so you'd have a good length of use out of them. If you are open to this option then I would recommend you look specifically at Axkid stockists, as most retailers don't stock the larger rear facing seats.
If you get massively stuck for space you could put the 6yo in the centre on a Bubble Bum, as they will have a bit of side impact protection just by virtue of being in the middle between two other seats, but it probably won't be as comfortable for them or support their head if they fall asleep.