A busy schedule

Guillermo del Toro is one of the busiest people in Hollywood, producing at least 6 major movies in the next couple of years, besides directing his own movies. So making Hellboy III would mean that he has to shelve one of his projects.

And this has happened already. Guillermo del Toro was in talks with Warner Bros for a Justice League Dark film, to appear sometime after Pacific Rim II (so, maybe, in 2019). However, the studio asked from him to hasten the project, for a 2017 release date "“ the same year in which Pacific Rim II will be released.

Having written already the script for Justice League Dark, the director had to make a choice "“ and he chose the latter. So his schedule is free after 2017. All that is needed is for the fans to go see Pacific Rim II. And Hellboy III will happen"¦