Where are Heather and Windshear during the events of How to Train Your Dragon 3: The Hidden World?

In short, the various 'Dragons' TV shows are largely bound by the events of the films, but not vice versa. The makers of the films are very well aware that the movies are for a mass audience (HTTYD1 was seen by more than a hundred million viewers worldwide) and the TV shows, frankly, are not. By comparison, an average episode of Dragons: Race to the Edge might be viewed by a million or fewer watchers worldwide.

This means that events from the movies need to remain the primary canon whereas new characters (such as Heather) and elements (such as Windshear) created by the makers of the TV shows are often dumped when they make subsequent movies, moving them into a status that is often referred to as secondary canon.

BS: Let’s go off topic for a little bit with a question for Dean. There is almost always an ongoing argument within the Dragons fandom over what can be considered canon or not if it’s in any other medium outside the feature films and shorts. Even the TV series has its ‘canon’ status called into question on a regular basis. In your own opinion, what do you consider to be the official canon for the How to Train Your Dragon franchise?

DD: The feature film trilogy and the characters contained within it serve a narrative purpose specific to those three films, but we’ve made efforts to ensure that every expansion, whether it’s in the TV series, comics, or other mediums, have a sense of tonal consistency and storytelling unity in keeping with the feature films. The only real exception is Cressida Cowell’s book series, being that her storyline focuses on a younger Hiccup and his talking, dog-sized dragon named Toothless. The feature films were a conscious departure from Cressida’s books, in order to tell a story that had more of the tropes of a fantasy adventure. So, within the world of the films, we have tried to remain consistent in all of the expansions. The comic books will adhere to the same constraints and tone of the trilogy. The TV series and comics are meant to fill in time jumps between the films, offering insight and back-story to compliment the main narrative of Hiccup’s coming-of-age."

_{^{‘How to Train Your Dragon’ Graphic Novels: An Interview with Dean DeBlois and Richard Hamilton}}