Three Sisters Volcanoes in Oregon, by Lyn Topinka, United States Geological Survey

Bad things, as the saying goes, come in threes. So do the fates.

In Greek mythology, the fates are called "Moerae" or "Moirai." (The word moerae means to apportion or divide.) There were three, described using metaphors drawn from weaving. The poet Hesiod is likely the source of their names:

These are Clotho, Lachesis, and Atropos, and they give mortals their share of good and evil.

— Hesiod, Theogony, 905-6

Each of the three has a separate role. Clotho, who spins the threads of human life, Lachesis, who measured its span, and Atropos, who cut it with a shears. (Atropos is the eldest, a wizened creature who, of all the fates, is most feared. Her name, meaning inexorable or inevitable, is the source of the name for the drug "atropine," a belladonna derivative that relaxes, or paralyzes, muscles by interfering with nerve conduction.)

Meanwhile he'll not suffer harm or trouble, not before he sets foot on his own land. After that he'll undergo all those things Destiny and the dreaded spinning Fates spun in the thread for him when he was born, when his mother gave him birth.

— Alcinous, The Odyssey, Homer, Book 7, line 240

The fates predate the gods of Olympus, and their relationship is much like the United States Constitution's balance (separation) of powers. She who cuts cannot measure. She who measures cannot alter the thread (events) of life. She who creates the life cannot determine its length. And so it goes.

Behind the power of the gods and beyond all the efforts of men, the three Fates sat at their spinning. No one could tell whence these sisters were, but by some strange necessity they spun the web of human life and made destinies without knowing why. It was not for Clotho to decree whether the thread of a life should be stout or fragile, nor for Lachesis to choose the fashion of the web; and Atropos herself must sometimes have wept to cut a life short with her shears, and let it fall unfinished. But they were like spinners for some Power that said of life, as of a garment, Thus it must be. That Power neither gods nor men could withstand.

Old Greek Folk Stories Told Anew by Josephine Preston Peabody, 1897

The Romans called the fates Parcae or Fatae; there were Nona, Decuma, and Morta: Nona, like Clotho, spins; Decuma, like Lachesis, measures; and Morta, like Atropos, cuts.

The Norse had the three Norns who wove human life: Urth or Wyrd, for the past or fate; Verthandi, for the present or necessity (of dying); and Skuld, for the future or being. When the end of the world arrives, Skuld will lay death upon the universe itself; in the meantime, they weave a tapestry of such unimaginable complexity that it will never be finished. The Norns live in a cave at the base of Yggdrasil, the world tree — situated in the Nornenberg (Nuremberg) mountains — where they try to stop its decay by pouring mud and water from the Well of Fate over its branches. (It is interesting, if not ironic, that the trials of Nazi war criminals were held at Nuremberg.)

The three weird sisters of destiny in Shakespeare’s Macbeth has the three weird sisters of destiny; these are the Scottish equivalents of the Moerae, the Parcae, and the Norns. (Weird is derived from "wryd," the Anglo-Saxon word for fate.)

First Witch: Thrice the brinded cat hath mew'd.
Second Witch: Thrice and once, the hedge-pig whin'd.
Third Witch: Harpier cries:—tis time! 'tis time!
First Witch: Round about the caldron go;
In the poison'd entrails throw.—
Toad, that under cold stone,
Days and nights has thirty-one;
Swelter'd venom sleeping got,
Boil thou first i' the charmed pot!
All: Double, double toil and trouble;
Fire burn, and caldron bubble.

MacBeth, by William Shakespeare, Act 4, Scene 1

David Brin wrote a science-fiction novella, "The Loom of Thessaly" (published in The River of Time, 1981, but available online), about the fates. I quite liked it, but YMMV.