The entire buildup of season 3 is Jim's training to kill the series' villain with extreme prejudice.

That already sounds like an action serie. Even though I didn’t see any of three series of Trollhunters, it sounds like it’s worth watching it. I assume it has a great animation when it comes to graphic as well. Your review sounds exciting right from the beginning. I personally don’t mind spoilers, I know many people hate to know ahead, but I take it as a preview. And thanks to this preview I can decide whether it’s worth watching.
Great review!