These are tricky ages. Have you looked to see whether there are any baby equipment hire firms near your destination, often they can be cheaper for car seat hire than doing it directly through the company, and sometimes (this is really variable) they can be better quality seats and better cared for, or you can at least ask about the exact models and look up the instructions online etc. Car hire places are notorious for giving you absolutely dire seats - you could end up with backless boosters for the older two and some ancient relic for the 1yo with no instructions as to how to fit it safely.
The other suggestion to look at is whether there's a Carrefour, Leclerc, Bébé9, Orchestra or similar near the airport. Take a seat for the youngest, sit older two with just seatbelts and drive straight to the supermarket/baby shop and get 2 of the cheapest high back boosters (in fact, I would recommend getting one, coming out into the car park and checking whether you have any hope of fitting a second one in before you buy 2.) You might be able to use click and collect, but when I tried to do this in Spain they wanted an address and it was too complicated so I gave up, but it was easy enough to check in-store stock on the website anyway.
I would take an infant carrier for the 1yo if he fits in one. Kiddies Kingdom have Britax Babysafe Core for £50 at the moment which is a really good price and it's a generously proportioned seat. It fits up to 87cm / 13kg, which is probably bigger than you'd assume. A 1yo's legs will stick out but this is not a problem at all, as long as he is under the 87cm and his head is contained within the shell, it's safe for him to use it. This is compatible with pretty much any pram as well with "universal" car seat adapters which you may already have. An old infant carrier is sometimes the kind of thing people have around in their house which they might be happy to lend out. Again, you need to check his head fits into the seat shell and he's not over the weight limit, or height limit if it has one.
If he is already over 87cm, Halfords do a £60 seat called Impala which is belt fitted and RF up to 105cm but it's a full sized seat, so not that portable. Lighter than a spin seat, but that's it. Cheaper than any of the Joie options.
There are some seats on the market now which are FF with 5 point harness which fold up and fit using isofix and top tether, IMO these are a better bet than the Maxi Cosi Nomad which is seatbelt fitted and doesn't get a good fit in all cars. Cosatto On the Go, or Kinderkraft Fix2Go are two of these. Maxi Cosi do have a Nomad XL which is the same idea, but it's more expensive and I can't see any reason to pay more. These are basic seats but they are a game changer when it comes to travel/harnessed seats - there hasn't been anything really like this before on the market. Depending on size/age of your 1yo, might be worth a look, though I would defo investigate RF options first.
The harness on those seats only goes up to 105cm, so unless your 5yo is very small for their age, they probably won't fit into one and I am not sure I would bother with one just to use it as a HBB.
You can buy folding HBBs in the UK e.g. Maxi Cosi Tanza, but they are still fairly heavy/bulky and I think probably a bit much for a 7yo to manage especially if they are tired after travelling. So if at all possible, I would look to find something you don't need to transport around.