The most popular type of three wheeler is the VW trike. It is very easy to convert a VW bug -- a.k.a. VW Beetle. The rear engine makes the build easy and the highly reliable air cooled motor simplifies the mechanical aspects. Basically to build a VW trike you need a VW Bug, a motorcycle front end, steel to build the connecting frame, and mechanical and fabrication skills.
One of the key points in building a VW trike is design. So many home-made VW trikes look like dumpy Mexican ice cream vendor carts. You really want a sharp chopper looking design. You can either fabricate a steel body or use a fiberglass body but the basic build -- e.g. rake, length, etc. --needs to be cool.
One of the other great advantages of the VW trike is that the rear weight makes it real easy to do wheel stands -- ABOVE. VW trikes are also very capable of towing a utility trailer or camp trailer.
...and don't forget, if you get caught in the rain on a VW trike you may get wet but you'll be able to keep going.