Cerberus was an inhabited planet in the Cerberus system. This planet was located somewhere in the Alpha or Beta Quadrant.
In the 23rd century, this planet was home to the Federation Cerberus Colony. (TAS: "The Survivor"; SNW: "Children of the Comet")
This planet was only mentioned in dialogue.
StarTrek.com stated that the the planet was named after Cerberus, the three-headed dog that guarded the gate to Hades in Greek mythology. [1](X)
In the Star Trek: Star Charts ("United Federation of Planets I"), Cerberus was listed as a Federation member in 2378.