This question has been closed as general reference: Who is the cat in chapter 17 of Prisoner of Azkaban?
I think the question deserves downvotes but not close votes. If you see comments there, CVers have chosen Wikia and Google as a ground of general reference which is wrong as per community standards.
Why was the question closed? Should it have been?
The Wikipedia entry for Prisoner of Azkaban describes clearly the relevant plot from chapter 17.
While at his cabin, Hermione discovers Scabbers in Hagrid's milk jug. They leave, and Buckbeak is executed. As Ron, Harry, and Hermione are leaving Hagrid's house and reeling from the sound of the axe, the large black dog approaches them, pounces on Ron, and drags him under the Whomping Willow. Harry and Hermione and Crookshanks dash down after them; oddly, Crookshanks knows the secret knob to press to still the flailing tree. They move through an underground tunnel and arrive at the Shrieking Shack. They find that the black dog has turned into Sirius Black and is in a room with Ron. Harry, Ron, and Hermione manage to disarm Black, and before Harry can kill Black, avenging his parents' deaths, Professor Lupin enters the room and disarms him. Harry, Ron, and Hermione are aghast as Lupin and Black exchange a series of nods and embrace. Once the three students calm down enough to listen, Lupin and Black explain everything. Lupin is a werewolf who remains tame through a special steaming potion made for him by Snape. While Lupin was a student at Hogwarts, his best friends, James Potter, Sirius Black, and Peter Pettigrew, became Animagi (humans able to take on animal forms) so that they could romp in the grounds with Lupin at the full moon. They explain how Snape once followed Lupin toward his transformation site in a practical joke set up by Sirius, and was rescued narrowly by James Potter. At this moment, Snape reveals himself from underneath Harry's dropped invisibility cloak, but Harry, Ron, and Hermione disarm him, rendering him unconscious. Lupin and Black then explain that the real murderer of Harry's parents is not Black, but Peter Pettigrew, who has been presumed dead but really hidden all these years disguised as Scabbers. Lupin transforms Scabbers into Pettigrew, who squeals and hedges but ultimately confesses, revealing himself to be Voldemort's servant, and Black to be innocent. They all travel back to Hogwarts, but at the sight of the full moon, Lupin, who has forgotten to take his controlling potion (the steaming liquid), turns into a werewolf. Sirius Black responds by turning into the large black dog in order to protect Harry, Ron, and Hermione from Lupin. As Black returns from driving the werewolf into the woods, a swarm of Dementors approaches, and Black is paralyzed with fear. One of the Dementors prepares to suck the soul out of Harry, whose patronus charm is simply not strong enough. Out of somewhere comes a patronus that drives the Dementors away. Harry faints.
The Cat, Rat, and Dog clearly listed in the Wikipedia article are Crookshanks, Scabbers, and Black. Unless we are to make the logical leap to assume that Crookshanks transubstantiated himself into McGonagall or some other Cat within the Shrieking Shack, it clearly answers this question.
