A car light enough that one of two people could pick it up would have a prayer, but it wouldn't be easy. Though I'd be VERY concerned about a three-wheel design for unrelated (safety) reasons. Three-wheelers are substantially easier to tip over than four-wheelers.
Heck, the Gossamer Albatross made it around the world (very slowly) using a fraction of a horsepower. Sure, it's possible if you're allowed to bend the conditions beyond where the market will let you. No one is going to fly ultralights commercially for a number of reasons (not the least of which is that it's much faster to drive).
I don't think microcars are likely ever to be commercial successes, for similar reasons. But that IS the direction one has to go if one is to get big improvements in mileage.