In the realm of modern human-pedaled and electric-assist tricycles (e.g. for carrying kids, passengers, cargo, etc.) the single wheel at the back configuration is called a "tadpole" and it's common and considered stable. Caveat: "stable" is relative. Example: Bicycles are unstable at rest but stable at speed, compared to any tricycle which is stable at rest but unstable at speed, particularly when turning. But plenty of fun if you're willing to lean. See also: professional motorcycle sidecar racing. :-)