Having a "Age of three powers" is easy. Just create a world with only 3 - either titan or megabeast- great creature.
You will first see "age of three powers", then "age of XX and XX", and after "Age of -nameofthecreature" or "Age of raceofthiscreature" or "The nameofthecreature-ic age" (For the titan, i haven't check wether it's "Titanic age" ? ^^ Seems to work only with dragon and not with Roccish, or Hydric)
After the age of myth and legends, there SHOULD be a "three power" or "age of XXX", when it's only 2 or 3 megabeast creature.