The third sacral dorsal rami, which are smaller than the dorsal rami above, exit the sacral canal through the third dorsal sacral foramina between the fused third and fourth sacral vertebrae.

They divide into medial and lateral branches, emerging deep to multifidus.  The medial branches terminate in mutifidus, which they supply.  The lateral branches pierce multifidus and the aponeurosis of erector spinae to terminate in the skin over the sacrum and medial gluteal regions.

Supply
The third sacral dorsal spinal rami supply multifidus and skin over the sacrum and medial gluteal regions.