The original How to Train Your Dragon animated trilogy contains three feature films.
The films are How to Train Your Dragon, released in 2010; How to Train Your Dragon 2, released in 2014; and How to Train Your Dragon: The Hidden World, released in 2019. Together they follow Hiccup from an uncertain young Viking to a leader responsible for Berk’s future.
The franchise also includes television series, specials, short films, books, games, and a later live-action adaptation. Those additional productions are not counted as part of the original animated feature trilogy. This is why the answer is three rather than the larger number of screen projects connected with the franchise.