It comes as no surprise that the three headed hound belongs to Rubeus Hagrid who is renowned as having a penchant for collecting dangerous beasties. When being verbally grilled by the three young wizards, Hagrid boasts about how he acquired the guard-dog from a “Greek chappie” he met in the pub. It then becomes clear that Fluffy is intended to be Cerberus of Greek Mythology – the three headed dog which guards the gates of the Underworld. (Harry Potter and the Sorcerer's Stone)
Old limestone steps leading into a Root Cellar; a subterranean place of cool and constant temperature where perishable food may be safely stored. In a dream the downward stair beckons you further into yourself.