According to Mignola Hellboy 3 won't happen
Although Mignola did not come with any particular topics in mind, he did make one thing very clear: there will be no "Hellboy 3" film.
Though, I'd be more apt to believe Del Toro, as he's the one making the movie. Maybe I'm just in denial, but I really really REALLY want Hellboy 3. Hellboy is among my all time favorite movies.