Three (The Ark)

Three was a Monoid who lived on the Ark in the 57th segment of time, seven hundred years into the ship's voyage.

When the Ark arrived at Refusis II, Three talked with One about what should be done with the Ark's humans. One told Three he had hidden a fission bomb on the ship to kill the humans once the Monoids were settled on Refusis. Three also warned One that he believed that Four was planning a mutiny. Three was correct and was killed during Four's mutiny. (TV: The Ark)

Behind the scenes
Of the Monoid actors (Edmund Coulter, Frank George, Ralph Carrigan, John Caesar), it is never made clear who played which Monoid.