The three-speed bicycle is a bicycle that uses internal hub gears at the rear wheel hub to provide three gear ratios. Typically, in low gear, the rear sprocket turns faster than the wheel; in middle gear, the rear sprocket turns at the same speed as the wheel; in high gear, the rear wheel turns faster than the sprocket.
Three-speed hubs have been in use since the early 1900s. Though they are heavier than comparable derailleur systems, internal-gear hub systems generally last longer and require less maintenance. During their heyday, three-speed hubs were used in all applications, including racing and time trialling, cargo cycling, utility cycling, and commuting. In many parts of the world, three-speed hubs are still preferred for commuting and other types of utility cycling over derailleur systems.
Internal-gear hubs come in a variety of models, including three-, four-, five-, seven-, eight- and even 14-speed versions. Notable manufacturers of internal-gear hubs include Sturmey-Archer, an early pioneer (now owned by SunRace); Rohloff; SACHS/SRAM; and Shimano.
The Raleigh Bicycle company has built many classic three-speeds, including the Sports, Superbe, and Tourist.
The late Sheldon Brown has a website with many details about three-speed bicycles, including the history and workings of the bicycle.