BBC Three (in-universe)

BBC3 was launched in 1969, with its initial broadcast showing the British Rocket Group and Mars Probe 7. (PROSE: Who Killed Kennedy) Dame Emily Shaw saw her daughter Liz Shaw over John Wakefield's shoulder immediately before the launch of the Recovery 7 several days later. (AUDIO: The Last Post)

BBC3 broadcast episodes of Roland Rat: The Series. The Sixth Doctor once introduced an episode of the series, and fell asleep before the end of the episode. After the episode finished, the Doctor was awoken by the prodding of a pole. He recapped the viewers on what they had been watching, and then excitedly told them to switch over to BBC1 to watch Doctor Who, which he considered "the series". Ron Rat then jumped up, calling Doctor Who "rubbish", leading to the Doctor attempting to shoot him with a ray-gun. (TV: Untitled)

It broadcast a documentary on Devil's End, (TV: The Dæmons) enitled The Passing Parade: Live from Devil's End, (PROSE: "BBC 3 Schedule") prior to the uncovering of the Devil's Hump. (TV: The Dæmons) The show was presented by Alastair Fergus, and he was joined on air by Professor Gilbert Horner. (PROSE: "BBC 3 Schedule")

James Stevens contacted BBC3 to expose the secrets of the Glasshouse, and they sent a camera crew to where it was located. However, the Glasshouse was abandoned when they arrived. (PROSE: Who Killed Kennedy)

Jo Grant later watched a documentary about a cult in Utah on BBC3, which "gave [her] the creeps." (AUDIO: The Many Deaths of Jo Grant)

On the Inferno Earth BBC3 broadcast the documentary series Occult Secrets of the Nazis. (PROSE: Still Lives)

BBC3 was the channel home to 60 Seconds. (PROSE: Have You Seen This Man?)

On 25 December 2010, BBC Three broadcast John Fuchas' film adaptation of The True History of Planets. (PROSE: Mad Dogs and Englishmen)

Behind the scenes
At the time The Dæmons was produced, only two BBC television channels were in operation, the existence of a BBC 3 was intended to suggest a future timeframe (see UNIT dating controversy).

In the real world 1970s plans did exist for a third BBC analogue network, but were ultimately shelved in favour of the IBA's second network Channel 4.

In the script for the ultimately unproduced 1993 film The Dark Dimension, a BBC3 news reporter named Tony McCabe is said to have been killed at Oxford Circus.