In Harry Potter and the Philosopher's Stone, the three-headed dog guarding the trapdoor is named Fluffy.
Fluffy belongs to Rubeus Hagrid, who acquires the enormous dog from a Greek man in a pub. At Hogwarts, Fluffy guards the trapdoor leading toward the Philosopher’s Stone, helping protect the Stone from anyone trying to reach it. Harry, Ron, and Hermione eventually discover that music can make Fluffy fall asleep, allowing them to pass.
The name is an intentional joke: Fluffy is a terrifying giant dog, but his name sounds like that of a cuddly pet. The character also echoes Cerberus, the three-headed guard dog of the underworld in Greek mythology. Cerberus is therefore an understandable answer thematically, but it is not the dog’s name in the novel.
Norbert is the dragon Hagrid secretly hatches, while Fang is Hagrid’s large but ordinary boarhound. Those creatures belong to the same early Hogwarts adventures, which is why they are easy to confuse with Fluffy.