Where did you get the Axkid seats from? If you have a trusted local retailer, I would try there as a first resort. If you got them online, try looking for any stockist of any car seat brand local to you and look for a non-chain, failing that find the biggest chain store that has a car park and go in and ask if you can try out different combinations of seats.
The Superb has loads of front to back space so did you have the Axkid seats on full leg room setting? They are allowed to slightly overhang the seats to enable this. 5/6 is obviously also perfectly reasonable to move to FF though.
With fitting three across it is much more about the shape of the seats than the overall width, so the only thing to do really is try things out. It's probably worth bringing at least one Minikid to the fitting session as well since an infant carrier won't last forever and your DC will still be needing HBBs when the baby is moving up to the next stage. The reason I'd say look at a big store or an independent is that they are more likely to have done it before and have an idea of which seats fit well together.
With high back boosters, the brand and features of them matter less because they make less difference than they do at other stages. Though I would want to ensure that the lap belt has a good position (crossing at the hips/very top of thighs) and that the seatbelt is reachable and DC are comfortable and have room for their legs to grow.
If you do like the Britax, Adventure and Discovery are the same but Adventure has no isofix attachments.
Sometimes it is worth putting one DC in the front and then in the back you can have middle / one side so when a second adult is in the car, they can go on the other side. This can be more comfortable than squeezing in the middle and it can make a difference how many seats you can fit together. Baby might be the most accessible in the front if the airbag can be disabled, if you put a high back booster child in the front, it should be the tallest and the seat should go as far back as possible, but check the car manual about the airbag.
There are still a few infant seats which have a base which can be fitted using seatbelt which can help with putting it in the front or middle (but check if a support leg is allowed in the middle). If you already have a pram with maxi-cosi type (universal) car seat adapters, you could check whether these seats are compatible with those, because they tend not to be the more well known brands.