There are three bones of the os coxae (hip bone) that come together to form the Acetabulum.

http://en.wikipedia.org/wiki/Acetabulum

There are three bones of the os coxae (hip bone) that come together to form the acetabulum. Contributing a little more than two-fifths of the structure is the ischium, which provides lower and side boundaries to the acetabulum. The iliumforms the upper boundary, providing a little less than two-fifths of the structure of the acetabulum. The rest is formed by the pubis, near the midline.