If something trebles or if you treble it, it becomes three times greater in number or amount than it was.

If one thing is treble the size or amount of another thing, it is three times greater in size or amount.

A treble is a boy with a very high singing voice.

In sport, a treble is three successes one after the other, for example winning three horse races on the same day, or winning three competitions in the same season.