If we have a third gender, what would it be? What does it look like?

Sex is biology, gender is cultural. Because it is a cultural idea, it can develop at any time to include more genders. There are some cultures which have recognised more than two genders for a long time such as Indian and Native American.

Eventually there will only be one gender

I learned recently that many Native Americans acknowledged five genders. I got dizzy and had to sit down.

I have no idea, but that would be confusing as hell. I would have no idea how they'd fit into the mating process

We have already third gender
Shemale

It's called Tranny