Triple 9 is a code that police use when one of the agents is in danger. When the rest of the police hear him leave what he is doing and goes out to answer the distress call of his partner, because the code is only used when there is a real danger.
This is well known by the cops who star in the film, policemen who are corrupt and are associated with criminals to commit a robbery using this distress code.
The police plan to set up a kind of theater in which they will send this code, saying that several agents are in danger. That will get all Los Angeles cops on their way to their location.
The thieves will be on the other side of the city, and thanks to the false call of distress in that part of Los Angeles there will be no police, which will allow them to steal without discomfort.