OT III is the third OT level invented by L. Ron Hubbard for members of the Scientology cult/sect. It includes the erupting volcano event pictured on the cover of Dianetics, which was caused by murderous galactic overlord Xenu as part of his plan to implant billions of thetans with false memories.
OT III is a secret and reading it will cause you to die of pneumonia. It is available on the Web.