It's not true that only 10 cars fit three across the back. It all depends on the seats and the combination. The seats look fairly flat on the Mondeo but the middle seat isn't wide - that's where you'll potentially struggle. Can you measure the width between where the middle seatbelt is attached and the buckle?
For twins and a 3yo you could potentially look at Multimac, it's one of the options, though it will really limit how long the twins can be rear facing which might not be what you want. I would not hold my breath on their "longer" rear facing seat as they have been saying that for ages. And the baby seat insert things aren't compatible with prams.
Other than that yes you need to check what you can have in the middle seat, and then a good idea is to make an appointment at a local baby store and take every car seat you currently own (including seats your 3yo has outgrown) to essentially play "tetris" with the seats and see what will fit.
What seat does your 3yo currently have?
For the babies you might be able to look at belt fitted bases for infant carriers like this is currently on clearance at Halfords and takes the Joie i-Gemm or i-Snug seats, which are compatible with many prams.
https://www.halfords.com/baby-and-child/car-seats/car-seat-bases/joie-i-base-isofix-car-seat-base-186308.html?stockInventory=undefined&plpPlacement=9
Long term, ERF seats like Axkid Minikid are helpful for fitting three across, because they are narrow at the base and the tethers let you move it forward or back which can help with fitting in around other seats. If your 3yo would sit rear facing, which a lot of them would find that exciting if you present it as something new and "special" - then she could go into this now which would be a safe option for her and then when the babies are growing out of their infant carriers, your eldest could change to a slim high back booster, one baby could have the Minikid and then you could either get a second ERF seat or something like Graco Extend R129 for the middle seat.
Or Graco Extend might work now for your eldest in the middle, if you don't think rear facing would work for her. A high back booster might be suitable at nearly 4 but some children won't be heavy/tall/sensible enough to manage it quite then.
All of the possibilities need checking before you buy because it's more about the shapes of the car seats and which bits stick out where than looking at widths etc. Where roughly are you in the country if you are comfortable saying?
Counter intuitively, it's actually safer to have a rear facing child in the front as long as you can disable the airbag. If you do put a child in the front IME it's more comfortable to use the centre rear seat for a child seat anyway and let the adult sit at the side.