Is a three wheel vehicle a motorcycle?

Short Answer: No.

Long Answer: No, it isn't- as with a bicycle ('bi' meaning two, in reference to the amount of wheels) a motorcycle must have only two wheels. If a vehicle has three wheels, it is either a tricycle (if it is not powered by an engine- if it is a tricycle it's more likely powered by a small child ;D) or if it is engine powered it is a car. The Reliant Robin is a good example- it is three wheeled, and while notoriously difficult to handle because of this, it is classified as a car.

Sorry for the long winded answer, just thought I'd be thorough!