Although Homer speaks of only one Gorgon, Hesiod mentions three: Stheno, Euryale and Medusa, who are sisters, the daughters of Phorcys and Ceto. Some myths have Stheno and Euryale being Gorgons from birth, while Medusa was changed into one by Athena, thus becoming the only one of the three Gorgons that was mortal. Other myths state that the three sisters were all beautiful women who were changed into Gorgons.

All three of them were turned into monsters. Medusa and her three sisters worked as priestesses in Athena's temple, and one day Medusa and Poseidon laid in her temple (she was either raped or it was willingly) and when Athena found out she turned them all into the hideous creatures. It is said that it may be that they were allowed to keep their attractive faces to lure men in--only to be turned to stone.

I have heard of a myth in which Athena gets so furious because the sisters are in her temple with Poseidon, she changes them all into hideous monsters. However, although she is mosr angry with Medusa (she is jealous,) all three sisters get the same punishment more or less.