I think as kids get older, it is nice to take a kid or two extra (I have two of my own). I haul an extra kid between 2 and 4 times a week.
This is certainly HIGHLY dependent on your area and activities though. I drive a small sedan, which is my preference now that we are out of car seats and boosters.
I can fit 5 total. My husband drives a large SUV (he does use it for work and does need the space and the "off-roadiness") that can fit 7. We said we'd trade off if I needed to haul extra kids. We've done that once in one year. I just don't need to haul THAT many kids, it turns out.
I also would really take stock if you'd need something big enough for three adults in the back. Really, is that likely to ever happen? In my circle, someone would just drive themselves, none of us would need / want pack into a single vehicle like that to go somewhere.