Inner Body says that the three parts of the deltoid muscle are the anterior fibers, lateral fibers and posterior fibers. Each band of fibers originates from a separate location.
The anterior bands begins at the clavicle, the lateral band begins at the acromion and the posterior bands form at the spine of the scapula. Inner Body explains that each band serves a different function. The anterior band flexes and medially rotates the arm. The lateral band allows the arm to move away from the body and out to the side. The posterior band contracts the arm to allow it to move backwards.