By chapter three Matt has been alone at the cabin for some time. He has learned to adapt to his situation and become somewhat comfortable with his solitude. Though survival will be difficult, he feels that he can succeed with the resources that he has. One of the most important ones for survival and food is his father's rifle. This is important to the main idea of chapter three because it disappears.
Matt notices a stranger approaching the cabin and an old trapper named Ben boldly approaches and looks around while chatting with Matt. While Matt is uneasy, he sees nothing outwardly dangerous right away and since Ben seems intent on staying the night, Matt makes the best of it and treats him like a guest. However, Matt remains alert and uneasy so he intends to stay up all night. He does not succeed at staying awake, falls asleep and Ben steals Matt's father's rifle right off the wall. Immediately Matt realizes that eating and survival will be more difficult.
The main idea of this chapter is that Matt warily trusts a stranger who visits for the night but his rifle is stolen making survival that much more difficult.