The biceps brachii acts as a class 3 lever to flex the arm. The deltoid also acts as a class 3 lever to abduct the arm, as does the teres major in adducting the arm. The triceps brachii, however, acts as a class 1 lever to extend the arm.
In other words, when you're bending your arm or swinging it toward your body or away from your body, it's a class 3 lever. But if your arm is bent and you're straightening it, then your arm is a class 1 lever.