There are three of us (all adults) and I don't fancy the idea of two in the front seat and one behind in a car - seems very anti-social.
well, we're a party of 3 (2A, 1C) and we did fine. (DW and DS were in the front, and I was alone on the back seat)
as far as "social" goes, the restaurant is loud (it's a drive-in with a movie showing, and loudspeakers by your "window", so not much of a conversation going on. You watch the movie and eat, it's fun that way
Is there other seating available ?
there are very few "camping table" on the outer perimeter of the restaurant for those who can't sit in the cars/limos (disabled, high chairs, and so on) but availability is not guaranteed.
and, no matter what, you'll all be glued to the big screen, and struggle hard enough to find your food in the dark, so socializing is a lesser concern
Has anyone else coped with odd number parties ?
for us it was a great experience. We were still able to share a little bit of "social life" during the meal (something along the lines of "could you give me the ketchup please"

) but this is definitely not a restaurant where you will be able to have a conversation with your fellow party members.
It's loud, it's dark, and there's a show going on onscreen.