The Treble refers to an accomplishment by a male student at Saint Anselm College who successfully sleeps with three different women over the course of the Thursday, Friday and Saturday night drinking/partying binge.
Many have done the "double" but very few have, if any have completed the Treble due to the prude quality of women at St. A's in addition to its High School style gossip amongst campus.
The legend is based off of the very infrequent accomplishment of the Treble in European soccer of winning the league, domestic and European cups. To date only 7 teams in European Soccer history have done this. It usually happens once a decade, hence the rarity and legend of the feat.
I can't believe it, he did the treble. Got the last one in Lowers last night.
Dude I almost got the treble last night but was cockblocked at the last moment.
Dude your cousin is a legend! He's done the double so many times, when is he gonna get the Treble?