The Three Powerful Witches is one of the legend told by Selina.
History
Selina tells the Trix that she has a way to let them go into the Legendarium, and tell them the story about the Three Powerful Witches.
"They had an insatiable appetite for power, and they hated everything good, especially fairies."
Magical Abilities
It's unknown what is the ability of the Three Powerful Witches, but it's most likely that Selina or Acheron made up this legend so the Trix can enter the Legendarium with their role as "Three Powerful Witches".