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