Below is an excerpt from a story I wrote a while back about third gender peoples of Polynesia - including their important cultural role, and a brief mention of Māhū of legend:
“In Native Hawaiian and Tahitian cultures (māhū) are third gender people with traditional spiritual and social roles within the culture. Māhū were particularly respected as teachers, usually of hula dance and chant. In pre-contact times māhū performed the roles of goddesses in hula dances that took place in temples which were off-limits to women. Māhū were also valued as the keepers of cultural traditions, such as the passing down of genealogies. Traditionally parents would ask māhū to name their children.”
"In Hawaiian or Kanaka Maoli culture, gender is not a binary concept. Kānaka Maoli acknowledged those who did not simply identify as male or female. The third gender is the māhū, or ‘the in-between.’ This Hawaiian term is used to characterize someone who embodies both kāne (male) and wahine (female) spirit. Many other Pacific Islander cultures share this understanding of a third gender. In Tongan, the term is ‘fakaleiti,’ and, in Sāmoan, the term is ‘fa‘afafine.’”
There are four famous stones on Waikiki beach known as Kapaemahu. They were placed there as a tribute to four māhū of legend who were said to have brought the healing arts from Tahiti to Hawaii long ago and have been there since before the 16th century.
Unfortunately, I don't know enough about this to truly answer your question in a meaningful way, but as a social scientist, the general rule of thumb is that myths and religion tend to reflect real life dynamics and vice versa. So, although biological sex is overwhelmingly binary (although not entirely - even intersex animals exist) cultures that tend to embrace more than two genders would likely have myths that reflect that - particular if such persons held important or even sacred roles.
https://www.ihs.gov/lgbt/health/twospirit/#:~:text=Two%2Dspirit%20identity%20was%20widely,Same%2Dsex%20relations.
"(Native American) Two-spirit identity was widely believed to be the result of supernatural intervention in the form of visions or dreams and sanctioned by tribal mythology. In many tribes, two spirit people filled special religious roles as healers, shamans, and ceremonial leaders." (emphasis mine)
Here's a link from Encyclopedia Brittanica about cultures with more than two genders. There are some similarities and some differences, depending on culture. I may have have misspoken about 7 genders, but 5 does seem to be somewhat common.
Edit: Here’s something else that I remembered from another story I researched and wrote. The topic of how gender is conceptualized in different cultures fascinates me. I hope to get a chance to look into it further at some point.
“For the Gerai of Borneo, rape is an almost incomprehensible idea, in part because their conceptions of both gender and intercourse are slightly different from Western understandings. Men and women are viewed as largely the same, and work rather than biological sex is what determines gender.”
https://medium.com/inside-of-elle-beau/rape-is-unknown-in-some-cultures-7c8629c4fa0f
