I'd be in favor of a third option, if all 3 boxes were completely voluntary.
However, I'm afraid if a third option were implemented, it would be meant for transsexuals who've had SRS, instead of non-binary people. Everybody would probably be forced to choose anatomical sex, with those who've had SRS expected to check 'transgender', and everybody else check their birth sex.
I'm afraid the more people transition, the more we move toward such a thing, given society's obsession with genitals.
Even those who 'accept' transgendered people feel we should 'warn' everyone what genital package we received at birth.