The Third Galaxy was the galaxy in which the planets Urbinia and Baralda were located. Urbinia's capital city, Kiria, was a seat of great learning throughout the whole galaxy, with a bustling intellectual community dedicating itself to all schools of science, politics, philosophy and culture.
In an alternate timeline, the Daleks entered the Third Galaxy with a functioning Time Destructor. They destroyed Baralda and invaded Urbinia. The Second Doctor averted the timeline by ensuring his previous incarnation arrived on Kembel to stop the Daleks. (AUDIO: Daughter of the Gods)