3 wheeled vehicles designed for two people can be made stable and safe. And many are. As a matter of fact, Morgan has just reintroduced a three wheeler very similar to the one it built in 1909 that’s very stable and has its own race class.
http://www.bing.com/images/search?q=morgan+three+wheeler+images&qpvt=morgan+three+wheeler+images&qpvt=morgan+three+wheeler+images&FORM=IGRE
Others, such as the Isetta (I drove on e of these) were very unstable. But, on the other hand, so were the early Explorer and the Blazer… very unstable and unsafe, that is.
I think it’s great that a motorcycle license isn’t required to drive a three wheeler in CA. I’m surprised, and I doubt if there are many states that follow that formula, but that’s the legal classification that I support. Currently, NHTSA, FHWA, and DOT all classify a Can-am as a motorcycle. And most states follow the federal guidelines in categorizing for licensing. However, it may be true that some states accept car licenses as qualification for motorcycles. Apparently CA has its own scheme.