Triumvirate

The Triumvirate was the name given to a group of three, highest among the Founders of Time Lord civilisation on the planet Gallifrey; (PROSE: Remembrance of the Daleks) more than one account suggested there were as many as six Founders in all. (PROSE: The Infinity Doctors, The Ancestor Cell)

History
The Gallifreyans who would become the Triumvirate overthrew the rule of the Pythia in an event known as the Intuitive Revelation. (PROSE: 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. (PROSE: 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 was also credited with ending the barbaric period of Gallifreyan history known as the Dark Time. (TV: 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. (PROSE: Lungbarrow) The Doctor received many of the Other's memories. (PROSE: Remembrance of the Daleks, Cold Fusion, Lungbarrow)

Legacy
The court who placed the Second Doctor on trial consisted of three Time Lords who, (TV: The War Games) by some accounts, formed the High Tribunal. (PROSE: Doctor Who and the Doomsday Weapon, The Quantum Archangel) One account specified that it consisted of the First Time Lord, Second Time Lord, and Third Time Lord, the three highest-ranking members of Time Lord society. (PROSE: Doctor Who and the Doomsday Weapon)

Behind the scenes
A shot in The Timeless Children's Matrix-enabled flashback to the foundation of Time Lord civilisation gives much attention to Tecteun, Omega and Rassilon forming a trio. Although they are not seen clearly in the televised episode, a publicity shot showing all three facing the camera was released with the caption "The first Time Lords". This evidently recalls the iconography of the Triumvirate, although the third man is now the Second Tecteun rather than the Other.