So you can sign as many pieces of paper that you wish- you can’t establish thereby the existence of a ‘third gender’.
Gov. Jerry Brown signed into law a bill making California the first to widely recognize being nonbinary — neither male nor female — as a third legal gender option.
I don’t mean to sound crude, but if you are uncertain as to your gender, pull your pants down and any 5 year old can tell you.
Now whether or not you identify as that gender is a matter for psychologists. But that you exist, in se, as that gender, is, quite frankly, indisputable.