Final answer:
The three-headed dog Fluffy in 'The Sorcerer's Stone' is an allusion to the mythological three-headed dog Cerberus who guards the gates to the underworld.
Step-by-step explanation:
The three-headed dog Fluffy in the book 'The Sorcerer's Stone' is an allusion to the mythological three-headed dog Cerberus who guards the gates to the underworld. This allusion is evident through the parallel between Fluffy guarding the Philosopher's Stone in the book and Cerberus guarding the gates to the underworld in mythology.