This comes with a depressing story: Anton Yelchin was the voice of the series protagonist Jim Lake, but in June of 2016 he died, pinned between his Jeep Cherokee and a brick pillar in a "freak accident." He was 27. Emile Hirsch recorded the voice of Jim Lake for most of the episodes of the third season of "Trollhunters."
Gunmar is loose in Arcadia, and building himself an army. New enemies and allies are found, and the final showdown happens. The hero is irrevocably changed by his coming-of-age. The end.
Comparison to "Avatar: The Last Airbender." "Avatar" is straight-up fantasy, "Trollhunters" is modern urban fantasy. "Avatar" is hand-drawn animation on a relatively low budget, "Trollhunters" is computer-animated and didn't have any noticeable budget restrictions. They both ran for three years, and had a story arc that stretched across the entire run (in both cases essentially "good" vs. "evil"). That's the baseline, now let's talk about the important stuff. "Avatar" laid on the life lessons with a heavy hand, often having someone explain why you should do this or that thing, while "Trollhunters" embedded the ideas in the behaviour of their characters. For example, while Jim occasionally admits to fear, when he has to fight (he tries to avoid it wherever possible, another good lesson) he nevertheless leaps. Toby is another story: he's a bit of a coward. But here's the thing: if his friends are in danger, he will always overcome that fear and fight for them. We can all feel superior to Toby because we aren't as much of a doofus as he is, but we can all understand his fear - and his dedication to his friends. One of Jim's great features is that given the slightest indication, he'll believe someone wants to do the right thing. And his belief in them will often make the difference in their choice. This way of teaching life lessons (ie. not giving direct voice to them) is more subtle and requires more work on the part of the writers, and having watched the whole series, I'm pretty impressed.
They awake Merlin in the last few episodes: as the person who created the amulet that turns Jim into the Trollhunter, he has always been put forward as the embodiment of good. So it's a pleasant surprise to find out that he's an arrogant ass with his own agenda that doesn't sit well with Jim and his friends' more idealistic views ... and they have to work with him anyway. Like Merlin, most of the show's characters don't show clear-cut black-and-white morality.
The show spends more time than I like on goofy interludes (ex. the "Breakfast Club" redux in the second season), but "Avatar" was exactly the same. Definitely a good series.