Triad

The Triad were the ruling authority of the Silurian people. The Silurians at Wenley Moor were led by a Triad consisting of Morka, K'to and Okdel. Okdel was killed by Morka after helping the Third Doctor find a cure for a Silurian plague he had unleashed, and Morka and K'to were killed when their caves were bombed by UNIT. (DW: Doctor Who and the Silurians)

One Triad, which included Icthar, Scibus, and Tarpok, freed their Sea Devil cousins and attempted to take over Sea Base 4. They intended to start a nuclear war which would wipe out humanity, but were all killed when the Doctor flooded the control room with Hexachromite gas. (DW: Warriors of the Deep)

The Silurian city beneath Cwmtaff was ruled by the Triad of the scientist Malohkeh, the leader of the warrior class, Restac, and the elder Eldane. Malohkeh was killed by Restac after she decided to exterminate the humans in the city, and Restac was herself poisoned by fumigation gases released by Eldane, who did not wish for the humans to be slaughtered. (DW: Cold Blood)