What is the main message of Chapter 3 of Freakonomics?
The main message of Chapter 3 is that there are times when what everyone assumes turns out not to be true. The authors look at a few different instances in which "conventional wisdom" turns out to be wrong.
The authors talk about how various people can have a stake in creating a given conventional wisdom even if it is wrong. They note how a homeless advocate had a stake in making people think the problem of homelessness was much worse than it really was. They also point out how the police had an interest in making people think that drug dealing was a much more profitable "job" than it really was.
So, the main point of this chapter is that people can be convinced of the truth of some conventional wisdom even if that conventional wisdom is false. This often happens become someone has a stake in pushing that conventional wisdom.