Cerberus

             According to Greek Mythology, Cerberus was the guard-dog of the Underworld and was owned by Hades. He kept the living out of the underworld and the dead from getting out. He's typically represented as a huge snarling dog with three heads with a serpent for a tail.
           The first movie that comes to mind when I think of a three-headed guard dog is the first installment of Harry Potter. In Harry Potter and the Sorcerer's Stone, there was a Cerberus creature, ironically named Fluffy. He was a huge three-headed dog, guarding an underground chamber. The only way to get past him is to play him a lullaby. In Greek Mythology,  this is how the singer Orpheus got past Cerberus and got into Hades. So clearly the parallels are apparent.

                                         (Harry Potter and the Sorcerer's Stone, Google Images)