Without spoiling too much, the following arc goes even further into developing The League of Villains, finally introducing their leader and revealing his true plan. the league as a more immediate, tangible threat. The first arc of this season gives the new villains a chance to show off what they're made of, and set up

The plot takes a much darker tone this season, with The League of Villains newly-emboldened by Stain's actions in season 2, and full of fresh faces. This season picks up where the previous season left off, with the students preparing to go to an inevitably doomed summer training camp.

Many details that explain my opinion on this series in a broader sense are already covered there. Before this, I would recommend reading my reviews of the first two seasons of Boku no Hero Academia.