Same meaning as virginity, but relating to threesomes.
When you have a threesome for the first time you will lose your threeginity.
Bill: I finally got laid last night!
Ted: Really? Thats awesome!
Bill: And not only that, I lost my threeginity! Got 2 chicks back to my hotel room!
Ted: Whoa! How did you manage that?
Bill: I was playing Metallica loud and 2 chicks walked passed my balcony... I flexed my arms, and the rest is history