The Three Who Rule were three humans who had been mutated by the King Vampire. They were called Aukon, Camilla, and Zargo.
Biology
The three had been humans. After they were taken by the King Vampire, they became vampires and were given eternal life. They grew fangs and their skin turned pale, and they were unable to function in sunlight. They developed telepathic powers. Unlike the King, the three would only drink the blood of the living; they considered even the momentarily deceased to be unfresh. (TV: State of Decay)
Biography
Origins
The Three Who Rule were humans from the ship Hydrax, which had been drawn into E-Space by the King Vampire. They chose to become vampires in exchange for eternal life. They served "the Great One". They set up their ship as a castle with Camilla and Zargo as queen and king and Aukon as the royal councillor. They had guards take peasants from the surrounding village to be drained of blood for the King. They themselves drank the blood of the living.
The Time of Arising
For a thousand years, they waited until it was the Time of Arising. They took Romana as a sacrifice. However, the Fourth Doctor saved her by killing the King Vampire with a steel bolt from a scout ship of the Hydrax. The Three became their true age and crumbled into skeletons. (TV: State of Decay)
Clones of the three were later developed. (PROSE: Blood Harvest)
Other references
River Song noted the Three Who Rule as an example of vampires. (WC: Monster File: Vampires)