Triskelion was an inhabited planet. The planet was the homeworld of the Providers and the Thralls. This planet was located in the M24 Alpha system.
This world was ruled by a cabal of three powerful disembodied brains called the Providers. The Providers' society revolved around betting their monetary reserves (counted in a currency known as a quatloo) on various aspects of captured slaves' ("thralls") lives, including open combat with one another.
The Federation starship USS Enterprise under command of Captain James T. Kirk was obligated to visit the planet to secure the release of several crew kidnapped by the Providers. Upon arrival, the Enterprise discovered numerous species enslaved on the surface as thralls, including their missing crew. Eventually, to free the thralls, Kirk entered into open combat against three drill thralls, betting not quatloos on his victory, but the condition of freedom for all if he was victorious, or enslavement of the Enterprise crew if he lost. He won, and the Enterprise left the system shortly afterward.
Another condition of the wager was that the Providers would educate the remaining thralls and allow the development of a free society on the planet. (TOS: "The Gamesters of Triskelion")
In 2328, the Triskelion Orbital Station, in orbit of the planet, was a point of embarcation and destination for commercial transports. (TNG-R: "Inheritance")
In the 24th century the Cardassians did trade with Triskelion during the Bajoran Occupation, as did the Bajorans after they took control of Deep Space 9. (DS9: "Necessary Evil", "Resurrection")