That point gets stretched pretty good. Generally, anything pedaled gets welcomed onto the forums here, so the Utility and Recumbent forums will have quite a few posts about 3-wheelers. Before I got my first unicycle a year ago, I asked about it here, and got some very helpful answers. There COULD be a unicycle forum here; it wouldn't be out of place. There's just not.

A bit on the trivia side, but you can have 2-wheel unicycles- the wheels are stacked so that pedaling the top one turns the one below.