Are you a guy or girl?
Girl
Why don't you ask him? That must be the most simple and obvious solution
Does he seem the threesome type? How well did you know/date him before you "knocked boots"?
You should learn to COMMUNICATE. Ask him why he wants you to meet her. Seems sort of obvious he's got 3sum in mind but he too needs to communicate; although he did "open the door."
Threesome
wear something kind of cute, kind of sexy, kind of conservative. when you meet them, if they do anything to gesture a threesome, don't jump into it...let them lead you until you are totally sure that thats what they mean.