It's a group of three friends who always crash parties when only one of them is invited to it. When they hang out together they just play pool or N64, even if it's for hours at a time. They constantly refer to inside jokes in front of everyone and just laugh because they don't care. They sneak out to go to girl's houses, even if they stay they for only 10 minutes. Everyone knows a group of people like this because they brag about it all the time.
Mom: Everyone sign the birthday girl's card!
Girl: Holy shit, the Trifecta signed this in three different colors. They weren't even invited.