Harry Potter and the Philosopher's Stone

Further information: Plot of the novel
Harry Potter is a seemingly ordinary boy, living with his neglectful relatives the Dursleys, in Surrey, England. On his eleventh birthday Harry discovers from a mysterious stranger, Rubeus Hagrid, that he is actually a wizard, famous in the Wizarding World for surviving an attack by the evil Lord Voldemort when Harry was only a baby. Voldemort killed Harry's parents, but his attack on Harry rebounded leaving only a lightning-bolt scar on Harry's forehead and rendering Voldemort powerless. Hagrid reveals to Harry that he has been invited to attend Hogwarts School of Witchcraft and Wizardry. After buying his school supplies from the hidden London street, Diagon Alley, Harry boards the train to Hogwarts via the concealed Platform 9Â¾ in King's Cross Station.
On the train Harry meets Ron Weasley, a boy from a large but poor pure-blood wizarding family, and Hermione Granger, a witch born to non-magical parents. Once they arrive Harry and all the other first-year students are sorted between four houses: Gryffindor, Hufflepuff, Ravenclaw, and Slytherin. Because Slytherin is noted for being the house of darker wizards and witches, Harry convinces the Sorting Hat not to put him in Slytherin. He ends up in Gryffindor along with Ron and Hermione. Ron's older brothers were all placed in Gryffindor as well: mischievous twins Fred and George, Percy the prefect, Charlie (who researches dragons in Romania) and Bill (who works for Gringotts Bank).
At Hogwarts Harry begins learning wizardry and discovers more about his past and his parents. He gets recruited for Gryffindor's Quidditch (a sport in the wizarding world where people fly on broomsticks) team as a Seeker, as his father was before him. One night he, Ron, and Hermione discover a large three-headed dog named Fluffy (owned by Hagrid) on a restricted floor in the school. They later find out Fluffy is guarding the Philosopher's Stone, an item that can be used to grant its owner immortality. Harry concludes that his potions teacher, the unfriendly Severus Snape, is trying to obtain the stone in order to return Voldemort (who Harry encounters in the Forbidden Forest where he, Ron, Hermione, and Draco Malfoy are serving detention by helping Hagrid look for an injured unicorn after being caught wandering around at night) to a human form.
After hearing from Hagrid that Fluffy will fall asleep if played music, Harry, Ron and Hermione decide to find the stone before Snape does. They face a series of tasks that are helping guard the stone which include surviving a deadly plant known as Devil's Snare, flying past a swarm of bird-shaped flying keys and winning a dangerous, life-sized chess game.
After getting past the tasks Harry discovers that it was really Defence Against the Dark Arts teacher Professor Quirrell who was trying to steal the stone, and that Snape was protecting Harry all along. Quirrell removes his turban and reveals Voldemort to be living on the back of his head. Voldemort attempts to convince Harry to give him the stone (which Harry suddenly finds in his pocket as the result of an enchantment by the headmaster, Albus Dumbledore), by promising to bring his parents back from the dead, but Harry refuses. Quirrell attempts to kill him but Harry's touch prevents Quirrell from hurting Harry and causes his hand to turn to dust. Quirrell then attempts to take the stone but Harry grabs his face causing Quirrell to turn into dust and die. When Harry gets up Voldemort's spirit forms and passes through Harry knocking him unconscious before fleeing.
Harry wakes up in the school's hospital wing with Professor Dumbledore at his side. Dumbledore explains that the stone has been destroyed and that, despite Ron nearly being killed in the chess match, he and Hermione are both fine. The reason Quirrell burned at Harry's touch was because when Harry's mother died to save him her death gave Harry a magical, love-based protection against Voldemort. Harry, Ron, and Hermione are rewarded with house points for their heroic performances, and Neville Longbottom is rewarded for bravely standing up to them, winning Gryffindor the House Cup. Before Harry and the rest of the students leave for the summer, Harry realises that while all other students are going home, Hogwarts is truly his home.

Question: Who is Fluffy?
Answer: a large three-headed dog