I've been thinking on this topic, of building a 3 wheel commuter, for some time. From what I've researched, and with what I would want to accomplish, a reverse trike (aka 'tadpole'-2 wheels forward, one behind) design would be what I would build. I'd use a Geo Metro 1.0 L drivetrain up front and a welded tube chassis. Tandem seats.
Fiberglass body that would have a Cd of ZERO!
Bam, 100 MPG.
It would somewhat resemble a cross between VW's concept and the Vortex.
I think a welded tube chassis would be a lot lighter than all that plywood in the Vortex.