Origin: Anterior head (front): Anterior surface of lateral clavicle

Lateral head (middle):  Acromion process and spine of scapula

Posterior head (rear): Scapula, inferior edge

Insertion: All heads converge at deltoid tuberosity of humerus

Action:

Anterior Head(front): flexes and medially rotates the whole arm at the shoulder

Lateral Head (middle): Abducts the whole arm at the shoulder

Posterior Head (rear): Primarily hyper-extends shoulder, also involved in transverse extension

Comment