I think a third gender is a step back, like civil unions. If it is meant to include "other" genders (non-binary) that's fine, but if it is meant to segregate MtFs or FtMs, no thanks.
I was also surprised to learn (not here, I was researching this a few months ago) that Thailand actually doesn't have a lot of rights for transgender people even though it's a top destination for surgery for transsexuals.