Agreed, there are very few cars on the market that fit the bill. There's the Ford S-Max, Peugeot 5008, and the Audi Q7 (which is ludicrously big) - I think that's it.
It really does feel like a massive gap in the market, and I wonder why that is?
BTW, if you're still looking, and depending on the age of your kids, it might be worth looking at the MultiMac - it's basically a new back bench for your car, with 3x car seats built-in.
Maybe because not many people have triplets? My parents had five children and never had more than one baby seat in the car at a time (they only ever bought one baby seat).
You need to look at the Peugeot 5008 or larger MPVs if you want that.
It's a gap in the market, I'd kill for a Model 3 sized car which was 5cm wider and had three proper seats in the back.
Source: the last three months researching and looking for the smallest car I can buy which can fit the whole family in.