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These three muscles are often difficult to separate from one another in the cadaver and are usually identified as the levator
ani muscle.

Identify the ischial spines and locate the tendinous arch of the obturator internus fascia (tendinous arch), which runs anteriorly and superiorly from the ischial spines to the lower border of the obturator canal. This arch is a thickening of the obturator internus fascia and serves as an attachment site for the iliococcygeus muscle.

Links and References:
Grant's: 3.41
Netter (1ed.): 377, 337, 338 (2ed.): 373, 333, 334
Rohen/Yokochi: 323