One, the screenplay gets published, or two, it gets turned into an animated movie, just like last decade’s Hellboy: Sword of Storms and Hellboy: Blood and Iron, which were set in the continuity of the Guillermo del Toro movies.
Rest assured, we’ll let you know if either of those become a reality, however slim those chances are.
At this point, that leaves two more options on how we might absorb Hellboy 3 someday.