For your 3yo if safety is your top priority, you would be best off with one of the rear facing seats which last up to 125cm - Axkid Minikid, Avionaut Sky, Avionaut Stardust (if you like the swivel) or Britax Safe Way/Britax Max Safe. Rear facing will give you the best safety and these will last much longer than a Maxi Cosi swivel seat. The Britax ones are the cheapest when they get reduced.
Axkid/Avionaut are less well known brands outside ERF circles but they are trustworthy and make good quality products. With a 125cm limit rear facing seat, she could stay rear facing basically as long as you want, as she'd only size out of them at about age 7 by which point your eldest won't need a seat any more.
If you or she are not keen on rear facing then I would look at Maxi Cosi Titan i-size (various versions mostly comfort differences), or Britax Evolvafix if you want a bit more leeway weight-wise, these can both use the 5 point harness up to 105cm which should last you another year, then it converts to a high back booster. The Maxi Cosi one has a lower weight limit than the Britax but both are good trustworthy options. Make sure to use the top tether when in 5-point harness mode.
For the 8yo, I would probably go with one of two options. Either get a simple backless booster, because at nearly 135cm, he's likely to be close to outgrowing a lot of high back boosters and a backless booster will be completely practical and safe in the meantime, and it doesn't need to be specifically fitted to the car. These are also just handy to have as spares, e.g. for lifts.
Or, take him to a physical shop where he can try different boosters out (they often have a demo "car" seat) to see if any of them have growing room and are comfortable. However, bear in mind that with some cars, the interior roof at the sides is too low to extend some of the larger high back boosters fully for children over approx 135cm, so it might be best to get a cheap backless booster for him for now and then only actually buy a high back booster once you have the car so you can check it fits. In terms of brands, they are really much of a muchness at this point and usually the pricier brands are more comfortable or look better rather than offering advanced safety. Essentially the seatbelt is the safety device, the booster is just making sure it is positioned properly.
If you wanted to go for one of the longer lasting rear facing seats it's often best to look for a stockist near you, if you use the stockist search on Axkid's website it will usually bring up any nearby ERF specialists. They sometimes stock high back boosters too so could try some out here, otherwise a bigger branch of John Lewis, Mamas and Papas or possibly a very big Boots might have some to try out. Halfords is also not a bad choice because although they don't stock as many of the better brands for other ages, for high back boosters it doesn't matter quite as much.