This term is used when sharing a cigarette with a friend, In which the first party takes 3 drags then passes it to the 2nd party.
When more than 2 people are sharring a cigerette it is refered to as:
Ultra trays, ultra 3's, or just Ultra's
Hey Jake, Will you go Three's / Trays with me?
Dave: Hey Tim will you go trays with me?
Tim: Im allready going trays with Steve but we can go Ultras.