For some weird reason, there have been divine cliques of three for as long as anybody can remember. Like the Furies, these trios never leave each other's side. They come to school together, sit next to each other in class, eat lunch together, and pretty much only hang out with each other. It's really no wonder that these trios are so cliquish: a lot of times they're actually aspects of the exact same deity.
Medusa and her two sisters, Euryale and Stheno, get the award for ugliest trio in Greek mythology. (This is good for the Fates, because otherwise they'd be stuck with ugly prize.) The snake-haired Gorgon sisters were so horribly revoltingly hideous that they turned anyone who looked at them into stone. For tons more on these three, check out our files on Medusa and Perseus and Medusa.
The Hindu religion is based on the belief in two divine trios. There's the female tridevi which includes the goddesses Saraswati, Lakshmi, and Durga, and then there's the male trimurti made up of Brahma, Vishnu, and Shiva. Interestingly, the trimurti directly parallel the Moirae in terms of their duties. Brahma is the creator, Vishnu is in charge of life, and Shiva is responsible for death. Hey, if it works, why not stick with it?
This trio of Celtic/Germanic mother goddesses were mad popular back in the day, and tons of different versions of them were worshipped throughout Europe. Their likenesses were often carved into votives and altars, where they were usually shown as three middle-aged women sitting with baskets of fruit and/or animals in their laps.
Thought of as the Maiden, the Mother, and the Crone, the Triple Goddess is worshipped today by many Neopagans and Wiccans. Like the Holy Trinity of Christianity they are thought of as aspects of the same deity. This trio is a direct continuation of the ancient belief in goddesses that only travel in threes.