The Coven, the Tri-fold Goddess, and the maidens of Camelot.

Igraine was birth, Morgause was life, and Guinevere was death.

Igraine, Morgause, and Guinevere form a good starting point. Igraine was the first queen, the mother of Arthur and the literal starting place for the Arthurian legends. When Igraine moved out of the picture as the main female influence of the story, Morgause (or Morgana depending on the version) stepped up to the plate to become the leading woman. Morgause is the mother of the Orkney Knights and, more importantly, of Mordred, she created the greatest running theme in Arthuriana and kept the female presence in the mythos going strong. When Morgause is cycled out of the frame, Guinevere comes up and takes over as the third and final mighty queen in the legends. Though Guinevere is usually childless, she created the events leading to the death of Camelot, ending both the legends and the coven of queens.

Early Welsh stories assign Arthur three wives, all of whom were called Guinevere

Three separate wives, three indistinct women that later merged into the Guinevere we know. It was not uncommon in Celtic legends for a king to marry the land goddess to cement his claim to power. The land goddess would come in three forms, and the king would be married to all three, thus the three wives for Arthur. Guinevere’s exact origins (at least the mythological side of her character) aren’t clear, as she isn’t tied to any particular goddess we know of. The likelihood of the historical Arthur having three wives named Guinevere is questionable, and historians probably added this bit in to make Arthur seem more important/powerful and connect him to mythology. Whatever the origin of the Guinevere we have, it’s clear that she functioned as the land goddess for the Arthurian mythos. If Arthur lost Guinevere he lost everything. When Mordred married Guinevere in Arthur’s absence, it would have originally been less of a lust-driven act and more of a power move to wrest power from Arthur.

Arthur usually had three sisters, Morgause, Elaine and Morgana

The theme of Maid, Matron, and Crone come into play again with Arthur’s sisters, but age actually has nothing to do with their roles. Elaine is the Maid, she rarely appears in legend and she is relatively obscure to the mythos. She is gentle and doesn’t show any signs of aggression toward Arthur. The Matron is Morgause, she is the mother of a huge chunk of the main Knights of Camelot and identifies her power as much by her son’s worth as her own magical abilities. Morgause is manipulative and cunning, but she isn’t often portrayed as the evil sister. She’s never a pure entity, and often seems bitter toward Arthur, but Morgause is rarely the character trying to actively destroy him. Poor Morgana, the youngest daughter, is the Crone. She is often hard, shriveled and venomous; a cruel enchantress who is always trying to destroy Arthur’s life. Her sudden back-pedal into the vaguely dark, but overall trustworthy character we see later on in legend doesn’t move away from her role as the Crone. Morgana shifts from the vengeful witch to the wise, slow-moving Crone associated with positively aging femininity.

Lady Lyonesse, Lady Lynette, and Lady Laurel are another example of this three-fold feminity

These three ladies were the wives of three of the Orkney Knights, Agravain, Gaheris, and Gareth. Laurel is always married to Agravain, but Lyonesse and Lynette are switched between Gareth and Gaheris. (I prefer Lynette with Gareth and Lyonesse with Gaheris, but I digress) Lyonesse and Lynette are sisters, while Laurel is their niece, the same bloodlines, but different. Just as the trinity goddesses are.

There were also three ladies who went with Bors, Gawain, and another knight on a quest, each of whom BLATANTLY represented the trine goddesses. They were even aged properly! I don’t remember who went with who exactly, since I can’t remember the name of the third knight, but Gawain went with the maiden and I’m pretty sure that Bors went with the crone.

The coven, the three-fold concept of celtic femininity is a constant element in the Arthurian mythos. If there’s a female character, she probably belongs to a trine of some kind, a piece of the ancient concept of the goddess.