Okay, found a 3-wheel version is commercially available, called a Pedalo. Note that this is wheels and axle only, not a "tricycle" as such, so it may or may not be what you were thinking of. Also note these folks sell all manner of variants of it, but mostly a kid thing:
http://www.sport-thieme.de/rl/R=2106...-?intcmp=Smart
A movie of one of these, although a 2-wheel version:
http://www.youtube.com/watch?v=YQFsO6L3LZ4
"Pedalo" is also used to refer to pedal boats and maybe pedicabs, but 99% of the google hits for it are for the pedal boats.