How did Roaring Currents do--was it successful? Kind of surprising that they're committing to make the Noryang film before even seeing how the Hansan film does.
It was very successful, yes. It's the most successful film in Korean Box Office history.
The trilogy was planned shortly after Roaring Current's success, it just took 7 years for the other two films to come out because the director wanted to persuade Choi Minshik to reprise the role of Yi Soon Shin. He chose other actors to play Yi Soon Shin after Choi Minshik ultimately refused 6 years of persuasion.