It would help if you were more specific. I know its personal, but to only describe someone as ¨transgender¨ leaves a lot unsaid. Are you considering a girly girl with a penis (which I suspect is the case), a post-op trans girly girl (male to female) with a vagina, a guy with a vagina, a post-op trans guy with a penis, an ambiguous person, a trangender male to female lesbian, ... My point is that it's not clear to me what you are considering.

I would suspect that it would be the same as having a 3-way with a non-trans person except that the trans person might come with different parts than what you'd expect from a non-trans person.

And since you mentioned something about using terminology, I wanted to point out this resource which you may or may not be interested in: http://www.masstpc.org/media-center/transgender-101/ And I would like to caution, if you don't already know this, that most or all trans people like to be treated like the gender they are presenting and identifying as. It is practically always appropriate to treat a male to female dressed as a female, like any other lady.