Pelvic diaphragm is formed by the following muscles:

The pelvic diaphragm forms a broad muscular sling and provides substantial suppo to the pelvic viscera. This muscle group is comprised of the levator ani and the coccygeus muscle. The levator ani is composed of the pubococcygeus, puborectalis, and iliococcygeus muscles. The pubococcygeus muscle now is preferably termed the pubovisceral muscle and is subdivided based on points of inseion and function. These include the pubovaginalis, puboperinealis, and puboanalis muscles, which inse into the vaginal, perineal body, and anus, respectively. Ref: Leveno K.J., Hauth J.C., Rouse D.J., Spong C.Y. (2010). Chapter 2. Maternal Anatomy. In K.J. Leveno, J.C. Hauth, D.J. Rouse, C.Y. Spong (Eds), Williams Obstetrics, 23e.