A three wheeler is a vehicle with three wheels, either "human or people-powered vehicles" (HPV or PPV or velomobiles) or motorized vehicles in the form of a motorcycle, All terrain vehicle (ATV) or automobile. Other names for three-wheelers include Trikes, Tricars and Cyclecars. The term Tricycle is used somewhat interchangeably, but the term three-wheeler is more often applied to motor vehicles.
Many three-wheelers which exist in the form of motorcycle-based machines are often called trikes and often have the front single wheel and mechanics similar to that of a motorcycle and the rear axle similar to that of a car. Often such vehicles are owner–constructed using a portion of a rear–engine, rear–drive Volkswagen "Beetle" in combination with a motorcycle front end. Other trikes include ATVs that are specially constructed for off road use. Three-wheeled automobiles can have either one wheel at the back and two at the front, (for example: Morgan Motor Company) or one wheel at the front and two at the back (such as the Reliant Robin).
Three-wheeler cars, usually microcars, are often built for economy reasons, or as was the case in the UK, to take advantage of tax advantages, or as in the US to take advantage of the lower safety regulations, they are being classed as motorcycles. As a result of their light construction and often relaxed pollution reqirements, leading to higher efficiency, three-wheeled cars are usually very economical to run.
When the single wheel is in the front (the "delta" form, as in a child's pedal tricycle), the vehicle is inherently unstable in a braking turn, as the combined tipping forces at the center of gravity from turning and braking can rapidly extend beyond the triangle formed by the contact patches of the wheels. This type, if not tipped, also has a greater tendency to spin out ("swap ends") when handled roughly.
With two wheels in the front (the "tadpole" form or "reverse trike") the vehicle is far more stable in braking turns, but remains more prone to overturning in normal turns compared to an equivalent four–wheeled vehicle, unless the center of mass is lower and/or further forward. Motorcycle-derived designs suffer from most of the weight being towards the rear of the vehicle.
Due to the incidence of injuries related to their use, a 10-year ban was placed on the sale of new three-wheeled Bikes in the United States in January 1988.
In Pop Culture
In the TV series Mr. Bean, a Reliant Regal (a three wheeler) was often driving around town. The car was sort of a nemesis to Mr. Bean, as he would drive in such a manner as to cause the car to roll over and even refused a ride from the three wheeler when hitchhiking. 
- Auto rickshaw
- Bubble car
- Harley-Davidson Servi-Car
- Reliant Robin
- Volkswagen GX3
- Corbin Sparrow
- Three wheeled car
- Tilting three wheeler
- All about 3-wheeled ATVs
- Online A-Z of 3-wheeled cars
- Article on three-wheeled ATV safety from The American Academy of Pediatrics
- Gallery of Japan three-wheelers
- Theory and Practice of Three Wheel design. (English & Français)
- Photo catalog of past and present three wheel motor vehicles. (street driven)
- "Three Wheel Cars; Primary Factors that Determine Handling & Rollover Characteristics" at Robert Q. Riley's website.