Socra (The War Games)

The Doctor's trial was presided by the Third Time Lord, who formed the court alongside the First Time Lord and Second Time Lord; (TV: The War Games) in the same body, by several accounts, (PROSE: The Legacy of Gallifrey, The Three Doctors) he was the President's Chancellor during the First Omega Crisis (TV: The Three Doctors) and beyond. (COMIC: Return of the Daleks, et al.)

By one account, he was the final and only recorded incarnation of Socra. (PROSE: The Legacy of Gallifrey) By another, he was an incarnation of the Third Time Lord, who later regenerated into a new body. (PROSE: Doctor Who and the Doomsday Weapon)

As Third Time Lord
Alongside the councillors Goth and Adelphi, Chancellor Socra (PROSE: The Legacy of Gallifrey) was part of a council of three who tried the Second Doctor on the battleground-planet of the War Lords, eventually condemning him to a force regeneration and exile on Earth. He was also present for the War Lord's trial, and he watched as the War Lord and his guards were dematerialised. He escorted Zoe Heriot and Jamie McCrimmon to say goodbye to the Doctor before they were returned to their home time periods with their memories of the travels erased. (TV: The War Games)

As Chancellor
Socra was asked by Rassilon to look out for the Third Doctor during his exile on Earth, and convinced President Pandad IV to send another member of the High Council to Earth to warn the Doctor that had escaped their grasp and was now out for revenge. (PROSE: The Legacy of Gallifrey)

A Senior Time Lord, (PROSE: The Three Doctors) Socra was still Chancellor during the First Omega Crisis, when help from the first three incarnations of the Doctor was required. He objected to President Pandad IV's plan to have the Doctor cross his own time stream, citing the First Law of Time. (TV: The Three Doctors) At the end of the crisis, he insisted with the President that the Doctor had earned his freedom. When Pandad IV demanded a more candid explanation, Socra was authorised by Rassilon to reveal the existence of the CIA to him, though Pandad chose not to make it public so as not to weaken the population's faith in the Presidency. (PROSE: The Legacy of Gallifrey)

Fate
As Chancellor Socra, this incarnation died without regeneration when he was eaten alive by one of several of the Rani's mice, (PROSE: The Legacy of Gallifrey) in the same incident which triggered a regeneration in the Lord President. (TV: The Mark of the Rani, PROSE: The Legacy of Gallifrey)

By another account, the Third Time Lord regenerated into a new body earlier, during the Third Doctor's exile on Earth. (PROSE: Doctor Who and the Doomsday Weapon) After a report on the Doomsday Weapon went missing from the Time Lords' files, he was part of a group that decided to send the Third Doctor to the planet Uxarieus to investigate. (TV: Colony in Space)