I find the idea of a single, third gender to be a bit problematic. I don't, as a trans guy, think I belong to the same gender as trans women, so right there, three isn't enough. Once you allow that you need two, where do you stop? What about people who don't identify with just one gender, or with none?
A more practical goal, IMO, is to work toward making gender irrelevant for legal purposes. In today's society, the only thing I can think of for which gender status has actual legal consequences (at least in the US) is registering for the draft. In every other area, men and women are equal under the law, so why is the distinction necessary? (I'd argue that it would more equitable in any case to require everyone, or better yet no one, to register for Selective Service.)
If legal requirements for gender identification were removed, it would leave everyone free to identify as they wish -- or not.