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 an upcoming medical procedure. 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 are about to have sex with them, 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, their 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.