All cars are different and sometimes a large car does not have as much space for fitting car seats in the back than a smaller one.
We had a vauxhall omega estate, this very large car did fit 3 car seats in the back (the ones we had at the time).
However, so did our honda jazz!! This car is very small on the oustide, but a bit like a tardis. Also, the rear seat is one flat bench (not fancy moulded seats that can make fitting seats more difficult). I do suspect the inside of the door was much more “space saving” than on the omega. I was very surprised this fitted when we tried.
We now have a zafira, not bad and may fit 3 car seats in the back, depending on the seats.
Make sure you dont squeeze in too many seats and get one seat potentially interfering with the buckle on the one next to it.