Legally you can put the eldest in the middle with no booster seat, and at 8 this likely isn't disastrous, but it's not ideal if the lap belt is up on their tummy.
I think all cars have a proper 3 point belt in the middle seat now, even my sister's tiny Hyundai i10 does, what classes it as a full size seat is the width. For example we do have Peugeot 5008 and the three seats are totally separate and identical, it's not a bench, you can slide them back and forth. Most middle seats in a car though are 3/4 size width of a standard seat or sometimes even smaller. The Hyundai seat is tiny and when I've sat in the middle there I feel like I'm sitting on both seatbelt buckles at once. It would definitely not fit a car seat.
I haven't actually come across a lap belt in a car for a very long time, not a modern car anyway. I wonder if there was some law banning them in new cars at some point.
If you do want to fit three across, your best bet is to take the car and every car seat you currently own to either an independent car seat specialist, a big nursery specialist (like those warehouse places which always seem to be located in the middle of nowhere!) or somewhere like John Lewis and see if you can book a slot to bring everything and have a play around with different combinations. If your 4yo is OK in a high back booster, then you'll probably get 2x slim boosters (maybe one backless) and an infant carrier in side by side even in a standard car, and then a Swedish type ERF seat like Axkid Move/Minikid (which your 4yo could actually use now if you wanted to, though granted you might not want to have them rear facing if they are FF now!) - these are well placed to fit in, because the part where the child's feet are is very narrow.
It takes a bit of playing around (which is why it's best to book an appointment if you can) and fairly often it's more about the shape of how the car seats and the seatbelts fit together. In the 3008, we managed to get an isofix seat + baby seat on belted base + no-carseat teenager in a row but it wouldn't work in any other combination at all because the extra height offered by the base was what allowed the handle of the infant seat to clear the bottom of the frame of the toddler's seat. Tuck away the isofix of the boosters if you can, as this gives you a little more lateral movement. Most HBBs allow this. If you have any combination type seats (e.g. 123 type) it's worth swapping these for just high back boosters. The part you'll struggle most is if your 4yo is currently in a harnessed seat which attaches using isofix and top tether, because these are space hungry and tend to need to be right in the centre. But you could look at Cozy n Safe Hudson here which I think is a bit more compact than others of this type.
This is helpful even though a lot of these are older model seats now: https://erfmission.com/car-seats-3-across-in-the-backseat/