The Bo/Merlin/Morgana relationship is an excellent example of a One True Threesome done properly because it only truly works with all three of them;
Bo get the obvious benefit of lovers she can't drain by accident that she can also be (comparatively) monogamous with.
By her very nature, Bo also inspires Merlin and Morgana to look past the social standards that kept them from being together in canon, as Merlin falls for Bo as a creature of magic while Morgana bonds with her as a fellow noblewoman before Bo brings them both into her relationship with the other.
Viewing them all as people rather than creatures of magic, Bo provides an excellent balance between her two partners' perspectives; where Morgana rejects the establishment that would persecute her people, Merlin's most consistent goal since he became Camelot's defender has been to try and protect the innocent. Bo can get behind both goals in her own role as the unaligned succubus, putting her in a perfect position to help Merlin and Morgana find a compromise where they can pursue their own goals without it impacting their love for Bo and each other.
This is explicitly reflected for the first time when the triad meet to discuss Mordred, based on the dragon's warning that Mordred will kill Arthur in the future. Where Morgana wants to protect Mordred as a child who hasn't done anything yet and Merlin is concerned about the prophecy of his future as the dragon's warnings have been accurate for the most part, Bo suggests they get Mordred away from his current guardian and send him to live with a more benevolent group of druids who will hopefully stop him becoming the kind of man who would kill Arthur in the first place.
This continues after the triad travel to the Blood King's palace and learn that Bo is the heir to the kingdom and Morgana's mother was fae (having been stripped of her powers under circumstances that mean Morgana is now manifesting fae traits herself). Once again, each of them is well-suited to help the other adjust to their new discoveries, as Merlin provides Bo with support on discovering her greater heritage despite her common background while Bo can help Morgana deal with the realisation that she's not entirely human.
When Bo meets the younger Evony (who she knew as the Morrigan in the future), Merlin and Morgana each encourage Bo to consider this a chance to help Evony be a better person the same way she encouraged them to find a way to help Mordred be better.
Most importantly on a personal level, while Bo is the catalyst to bring them all together, Merlin and Morgana each come to recognise that they can't "depend" on Bo for everything. After Morgana learns about Merlin's magic, she and Merlin agree that they need to work on their feelings about him keeping his own abilities secret from her on their own, without Bo to act as a "mediator".
Bo's first sexual encounters with Merlin and Morgana each essentially reflect the role she plays in their lives;
Her first time with Merlin sees her physically dominating him as her dark side takes over to acknowledge him as a "worthy" mate, reflecting how her primary influence on him has been to help him gain the confidence to stand up for himself, such as physically confronting opponents rather than relying on his magic.
Bo's first time with Morgana starts with her offering Morgana comfort after Morgana learns about Merlin's magic, and she is subsequently relatively gentle with Morgana, just as she essentially appears to be helping Morgana hold on to her compassion and avoid the ruthless turn she took as the series unfolded.
As a result of Bo and Morgana's relationship, Uther begins making plans to arrange marriages for both women. This is a scenario that never came up in the show regarding Morgana, but the reason why it never came up is easy to explain; Uther had no reason to explore the possibility of a marriage for Morgana in the first two seasons, and after she returned to Camelot following her year with Morgause Uther was too relieved to have her back that he never even thought of making her leave the kingdom again until she finally made her new allegiance public.
Bo's influence on Camelot's existing defenders goes beyond just Merlin and Morgana, as demonstrated when circumstances force Merlin and Morgana to leave Camelot to find a way to save Bo's life. With Arthur now aware that magic isn't evil in itself, Merlin in particular expresses confidence in Arthur's ability to defend Camelot and be a good man even in Merlin's absence, particularly with Gwen, Lancelot, Kenzi and Gaius there to help him.
In Lost Kingdom, after Bo learns of her heritage as the heir to the Blood King, it is suggested that her status as the unaligned succubus will be an advantage if she ever takes the throne. When she explains to the King how she didn't choose a side after her trial by combat, Trick speculates that this will allow her to adopt a similar balancing role as his heir; where the Dark Fae distrust the Blood King as he is a Light Fae even if he keeps his council balanced, since Bo isn't aligned to either side she will be seen as a more neutral party.
The show was always relatively vague about exactly how Uther was able to kill the dragons during the purge against magic when it took Merlin becoming a Dragonlord just to get Kilgharrah to stand down after he was released. Here, it's revealed that this was unintentionally aided by a curse performed by the Blood King that punished Kilgharrah for his role in a failed underfae uprising. In other words, the entire dragon race was basically rendered extinct because they angered a man of great power with a revolution that didn't even succeed.
