People like the How To Train Your Dragon trilogy for multitude of reasons. Some like it for great animation and visual appeals, some like it for it's soundtrack. Some like it for the feelings it evokes. Some people like it for great writing — and this is something that HTTYD3 regrettably lacks to a great extent.
The story in the first movie was truly great. Almost every scene was vital, and the story is full of details — tiny little events that seem pretty minor at first, but end up having major consequences later down the line. The third movie, though, feels much more badly written. The moment you take a closer look to the story, it all comes crashing down.
A lot of people have written in great extent about why the third movie is flawed on many levels. This page serves as an index of the writeups, as well as a mirror in case any of those goes down. The reviews overlap a fair bit, but each covers the movie from a different angle.
(Side note: if you want to discuss these essays or express your dissatisfaction with How To Train Your Dragon: The Hidden World, feel welcome to join us on discord!)