Hi,
I have been asked to check internet for a 3 wheel motor bike for a neighbour but failed on google/yahoo to find what he is looking for. He saw one in US it is a long type motor bike with 2 wheels in back, it must have a name as searches just show up kids bikes.
I had a go with and seems to have thrown up a few results. Can't say for certain that's the official name/most common name/etc. but it might give you a start.
(Always wanted a go on one after seeing Billy Connolly spinning around on one)