RK, I think the difference is the cage that the car driver has around them.Consider a Reliant Robin, it's got 3 wheels and you can drive it on a bike licence. You wouldn't call it a 3-wheel bike though, you'd call it a 3-wheel car. It isn't just the number of wheels that makes it a trike, it's having handlebars and being exposed to the elements. IMHO of course.
Note, I'm not picking on Reliant Robin drivers, I was one before I passed my car test.