Hiya! The third volume was to have contained the "RPG" component.
All three volumes were planned no later than April 1974 (per John Snider's letter in Supernova detailing the contents) but presumably that fell
foul of TSR's plans to rip concepts and monopolize those without being beholden to others. In this case, Jim Ward appears to have been an innocent party given the manner in which he was encouraged to write a SF "RPG" by Gary.