HOW TO TRAIN YOUR DRAGON executed its unoriginal plot in a fun and touching way. The characters were likeable, the jokes were funny and the flying sequences were exciting. HOW TO TRAIN YOUR DRAGON PART 2 executed its unoriginal plot in a generic yet watchable way. The jokes weren't bad; just unmemorable. However, a certain scene happened halfway through that was dark in a good way (and not inappropriate for kids). It was the start of a new conflict that was complex, because it didn't have an obvious solution and because the characters were driven by their emotions rather than logic. HOW TO TRAIN YOUR DRAGON PART 3: THE HIDDEN WORLD executes its unoriginal plot in an uninteresting and at times boring way. The new characters (human or not) are bland, the jokes rarely work and America Ferrera's performance continues to be weak.
The humans and the dragons saying goodbye is a tearjerking moment and a mature way to end the trilogy... but it's undone a couple of minutes later when they reunite. The narration explaining that the dragons will remain hidden from the world until present day doesn't have the same emotional impact if the main characters of this franchise can visit them anytime they want.