What kind of people do you think the sheep represent in chapter 3 of Animal Farm?

The sheep in Animal Farm are very similar to sheep in real life. They're not bright and follow others easily. Napoleon teaches them quick and easy slogans to chant, and they quickly believe in what they are saying. He teaches them to break into the chant "four legs good, two legs bad" in chapter 5, but later on in chapter 10 when the pigs begin walking on two legs they eagerly mimic their new chant, "Four legs good, two legs better." While the other animals on the farm need more persuading, the sheep quickly accept Napoleon's various arguments without question. The reader can imagine people that are equally influenced and whose beliefs are as easily changed.
Since Animal Farm is an allegory (a text where the characters, events, and setting are used as symbols for a larger argument), it's a fair assumption that these sheep stand for more than just representing people who follow the crowd.
The sheep represent part of the propaganda machine of Stalin's Russia. They represent the people who believed all the propaganda presented to them by Stalin; instead of thinking for themselves and reaching their own conclusions, they repeat the slogans over and over again until they believe what is told to them and go so far as to tell it to others. The line between propaganda and the sheep's beliefs becomes so blurred by the end of the novel that it is difficult to see the difference between fact and propaganda.