In Chapter 3 of "The Graveyard Book" by Neil Gaiman, titled "The Hounds of God," the main idea revolves around Bod's adventure outside the graveyard and his encounter with ghouls. In this chapter, Bod is lured out of the graveyard by a group of ghouls who promise him a life of adventure. However, he soon realizes that the ghouls are dangerous and have sinister intentions. The chapter highlights themes of bravery, the importance of home, and the value of the protection and guidance provided by his graveyard family. Bod's experiences teach him about the dangers of the outside world and the importance of the safety and love he finds within the graveyard.