I guess you can but you'd have very little to no room to adjust seatbelts and so on. And while newer cars come with the full middle seat belt instead of a lap one, I don't think they really intended them to be used for a full sized car seat. I carried 2 full size car seats in my car (large wide estate) and it was a real squeeze for even an 8 year old to then fit comfortably in the middle due to the sides of each car seat boxing him in.
I'd think a 3rd full size seat would be a really tight squeeze and hard to get to if in a hurry. A booster seat in the middle of 2 full car seats leaves very little room also.
I'm a nanny and have looked after all number of kids over the years, 4 with the present family and I've never seen 3 full size car seats in a back seat yet with anyone. I'd probably never try it for myself either for safety and comfort reasons.
There are some great 7 seaters out there though. And tbh, you don't even need to have a people carrier anymore as a fair number of cars now are designed with 6 or 7 seats and look car like. Mazda make great ones, the Mazda 5 or you can have a Mazda 6 with extra seats in an estate version I seem to recall.
