One of my passions is the intersection between culture and psychology...here's a little sample of stuff I nerd out on related to a third gender that has existed for a long long time in a particular Mexican culture:
It has been suggested that in the Zapotec culture of Oaxaca, Mexico, there is a three gender system which predates Spanish colonization. In Zapotec cultures of Oaxaca, Mexico, a muxe is an assigned male at birth individual who dresses and behaves in ways otherwise associated with the female gender; they may be seen as a third gender. Some marry women and have children while others choose men as sexual or romantic partners.
Read more about it here: