Something that's treble has three parts or sections. You could describe a tricycle as a treble wheeled vehicle.
You can use the adjective treble to mean "triple," either because it's three times as large, or has three components. If you watch three movies in a row at a theater, you can call it a treble feature. A three-headed monster can also be described as treble headed. Another meaning of treble is a high-pitched singing voice, especially a boy's. Treble comes from the Latin word triplus, or "threefold."