saw the Polaris slingshot for rent when I was in Florida this year,looked like fun https://idrivescootersandtrikes.com/
Depends on the distance between the two rear wheels as to whether its classed as one or two wheels in the Construction & use regs and ergo whether its a Trike or a quad.
I've always thought a quad has 4 wheels, and a trike has 3 wheels..the names kind off give it away quad=4 tri=3 ..or am I missing something! when folk go on about a Quad bike that makes no sense as the bi in bike =2
The regs say that any two wheels where the center lines of the two contact patches are 460mm apart or less are to be classed as a single wheel. http://www.legislation.gov.uk/uksi/1986/1078/regulation/3/made item 7 explains it and its described as a three wheeled motorcycle not actually a "Trike".