You will need a reed pipe (flute) and the song from the Temple of the Muses. Playing the short song will lull the monster dog to sleep. Go right to the Temple Grove and enter the temple of the Muses....
Go to the temple of Hades and use the pomegranates to enter his realm. You will ride the River Styx to Hades' domain. When you meet Cerberus, play the song (blue-green-yellow-blue-red-green) and you...