Oh, please go on about the rule of 3 in Merlin
Now, most of this is probably my own thoughts, but if anyone else has noticed these, hey!
Okay, so we'll start with some easy ones. The Druids tattoo themselves with a triskele, a triangle of three joined spirals (I also have this tattoo) which can mean several things. The triskele has been used by a lot of people for a long time. It's still on the flag of the Isle of Man. In Celtic Ireland, it typically represented the cycle of birth, life, and death (see the de Bois siblings below). In the show's context, it's more likely associated with the Triple Goddess of the Old Religion (Maiden, Mother, Crone) who is represented both as three separate deities and as aspects of a single goddess (the Disir, for example, simply say "the Goddess").
Moving on, we (I) see a repeat of this pattern of threes. There are three High Priestesses - Nimueh, Morgause, Morgana. There are three Pendragons - Uther, Arthur, Morgana (and when Uther dies, we get Gwen, who becomes a Pendragon by marrying Arthur). There's also the famous Courage, Strength, and Magic triad. The infamous triangle of Lancelot, Gwen, and Arthur. The more show-specific triad of Arthur, Gwen, and Merlin. It's also very strongly implied that Mergwenthur was paralleled by Uther, Nimueh, and Ygraine pre-series (seriously, Nimueh and Uther's conversations in S1 were too intense for just friends). There's the three Disir. The three original Round Table knights who die - Lancelot, Elyan, Gwaine. And like I said, the three De Bois siblings - Ygraine, who died giving birth, Tristan, who returned from the dead, and Agravaine, who spent his life looking for revenge. The three people Merlin sent to Avalon - Freya, Lancelot, Arthur. The three loves who died in their partner's arms, the "hold me" trinity - Freya/Merlin, Isolde/Tristan, Arthur/Merlin.
Going a little deeper, there's also something to be said about the way there's a pattern of three in the main characters who use magic, coming back to the Triple Goddess. One on the "good" side, one "bad," and one kind of in between. In S1 that's Merlin (good), Nimueh (bad), and Gaius (middle), and this one specifically can also parallel the aspects of Maiden (Merlin), Mother (Nimueh), and Crone (Gaius). Then it moves a little in S2-3 so it's Merlin, Morgause, and Morgana in the middle. And then it moves again to Merlin, Morgana, and Mordred in the middle in S5. The only time these patterns don't really repeat is in S4, which is fitting because that's the season of change and also instability, because take one part out and the others are unbalanced (e.g. Uther dying, Lancelot dying, Gwen's banishment).
So there you go. The Rule of Three in Merlin, as seen by 0hheytherebigbadwolf.
