Triumvirate

The Triumvirate was the name given to the three founders of Time Lord civilisation on the planet Gallifrey. (DWN: Remembrance of the Daleks)

The Gallifreyans who would become the Triumvirate overthrew the rule of the Pythia in an event known as the Intuitive Revelation. (NA: Cat's Cradle: Time's Crucible)

Omega was a stellar engineer who created a device capable of changing the structure of stars. Called the Hand of Omega, this device was used to "customise" a star called Qqaba, but Omega was lost and presumed dead in the resulting reaction. (PDA: The Infinity Doctors)

Rassilon was an architect and law-maker who built upon Omega's experiments to provide Gallifrey with the Eye of Harmony, a singularity capable of providing infinite energy. Rassilon is also credited with ending the barbaric period of Gallifreyan history known as the Dark Time. (DW: The Deadly Assassin, The Five Doctors, Remembrance of the Daleks)

The Other was a mysterious figure who ultimately grew tired of wielding absolute power and allowed himself to become part of the genetic cascade which produced new Time Lords in the Looms. It has been speculated that the Other was then "reborn" as the Doctor. (NA: Lungbarrow)

This theory depends on the existence of the "Looms", devices which create fully-grown Time Lords in an effort to overcome the Gallifreyans' sterility. The new series of Doctor Who has shown the Master as an 8-year-old child and has made mention of Gallifreyan parents, so it's possible the Looms are no longer canon.