I think it's be a sequel because in the second one they left a major event unfinished (at least i think). Malificient and Pete i'm guessing beat the heartless, and malificient said she wanted that castle as her new base. The thing i learned about disney is they never have a bad of cliffhanger ending without ending it well the movie/game. Malificient is at full power and is in the Castle in the World that never was .and she did swear to have her revenge. I'm guessing she'll take part in III. Just a theory