Generally, a three-wheeled bike is not capable of going as fast as a two-wheeled bike due to weight differences and aerodynamic drag created by the shape. Three-wheeled bikes, also referred to as tricycles or recumbent trikes, have advantages such as being more balanced and stable than bicycles.
Many people find recumbent trikes to be more comfortable, including seniors who make up the majority of trike consumers. Although the seated position may be more relaxed, pedaling still requires a good degree of effort which makes recumbent tricycles a good form of exercise. The seated position doesn't encourage as much of a cardiovascular workout as a traditional bicycle, however there is less stress on the rider's knees and neck.