The three rulers:
In Roman myth three brothers Jupiter, Neptune and Pluto were each rulers of their worlds. Jupiter was ruler of the sky. Neptune was ruler of the sea. Pluto was ruler of the underworld.
The three graces were followers of the Goddess Venus. The three Graces gave beauty to the world. Every attractive human being could thank the three Graces for the gift of beauty. They chose the humans who would receive the gift of physical beauty.
The ancient Romans believed that the first seven vowels were invented by the three Fates. The first was known as "The Spinner". She spun the life web of the humans. Her name was Nona. The second was known as "The Apportioner". She decided how long each human was to live. Her name was Decima. The third was known as "The Inevitable". She cut the life when it was time for someone to die. Her name was Morta.
The three Furies were goddesses of revenge. They were greatly feared as they showed no mercy. They were particularly upset with anyone who killed a family member.
The entrance of a home was also protected by three different Gods. They had the power to keep evil demons from entering the house. The three Gods were Forculus-god of doors, Cardea-goddess of door handles and hinges, and Limentinus-god of thresholds.
Lastly there is the Goddess Hecate/Trivia. Three faced statues of Hecate were often found in forks of the road looking in three directions. She was the protector of witches, sorcerers and wizards. She was accompanied by ghostly and scary dogs. She was also associated with the three phases of the moon.
So there you have it. The number 3 is also significant to us Streghe.