Who were the three sisters who turned those who looked at them into stone?
The three sisters you are thinking of are known in Greek mythology as Gorgons and were called Euryale, Sthenno and Medusa. Not someone you'd want to meet for the first time on a blind date, Gorgons are monstrous feminine creatures with bodies covered in scales, hair of living snakes, sharp fangs, oversized tongues, and full beards. Anyone who laid eyes on a Gorgon would immediately turn to stone out of fright.
The best known of the Gorgons is Medusa, who figures in a number of Greek myths. Most famously, one tale tells that Medusa was once a beautiful woman, beautiful enough to vie with the Goddess Athena. Athena grew angry, some might say jealous, and decided to punish her by taking her best feature, her hair, and turning it into living snakes.
Regardless of how Medusa came by her petrifying locks, another young god, Perseus, definitively took them away. Aided by Athena, Perseus crept up on Medusa as she slept, and being careful not to gaze directly upon her, cut off her head in her sleep. From Medusa's blood sprang the giant Chrysaor and the winged horse Pegasus—she had been with child by Poseidon at the time.
Pursued by Medusa's sisters, Perseus fled with her head, using it to turn Atlas, who refused to aid him in his escape, into a mountain of stone. He then returned the Medusa head to Athena who placed it on her Aegis, a protective device. From that point on, the Gorgon head became a symbol of protection; Greek soldiers emblazoned pictures of it on their shields as an attempt to avert evil forces and put fear in the hearts of their enemies.