The pigs Snowball, Napoleon, and Squealer are leaders on the farm and in the beginning attempt to carry out the plans of Old Major.
Napoleon becomes a fierce leader who runs Snowball off the farm so that he can be in control of the other animals. He says that he wants to educate the animals, but he actually indoctrinates them with his own ideas. He orders the death other animals who profess to being Snowball's allies.
Snowball was the organizer who put the animals into groups based on their skills and abilities. He planned the defense against the humans to uphold the system of Animalism. After he is run off the farm by Napoleon, Napoleon uses Snowball as a scapegoat and blames many ills of the farm on Snowball.
Finally, Sqealer is the voice of propaganda in the story. He has a story for everything, and gives explanations to the animals for all occurrences on the farm. His aim is to keep the animals happy and to maintain their belief in Animalism.
In the allegory, the three represent Stalin, Trotsky, and propaganda, respectively.