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 daugher Liz Shaw over John Wakefield's shoulder immediately before the launch of the Recovery 7 several days later. (AUDIO: The Last Post)

It broadcast a documentary on Devil's End prior to the uncovering of the Devil's Hump. (TV: The Dæmons)

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)

Behind the scenes
At the time The Dæmons was produced, only two BBC networks were in operation, and the use of BBC 3 was intended to suggest a future timeframe (see UNIT dating controversy). In the real world, BBC Three launched in 2003 as a digital channel.