Winx Club Forever
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.”
It is unknown what is the ability of the Three Powerful Witches, but it is most likely that Selina or Acheron made up this legend so that the Trix can enter the Legendarium, with their role as "Three Powerful Witches". It was supposedly used to strengthen the Trix's powers and let them enter the Legendarium World.