What's That Third Sex Called?

Okay, so there are males, females and ... what else? What should we call them? Somebody should come up with a word for it.

This morning at a doctor's office I had a conversation with a nurse about cancelling my scheduled test due to lack of need. The person was either a man who had changed into a woman or a woman who had changed into a man, I couldn't tell. 

I know it's not supposed to matter and people are people and blah, blah, blah, unless of course we were about to have sex, but still it's disturbing and more than a little distracting. I'm pretty sure I missed some important instructions while I was busy trying to figure out if it was a he or a she. Either way, the final outcome was not good since I noticed.

So what can we call them? Shemales? That might work. Anyway, I suppose over time we will all just take it for granted that we have three sexes here on Earth. Until then, I remain puzzled.