I'll be honest, I haven't seen the first two parts, I just didn't know they existed, and maybe that's why I didn't like the movie so much. Perhaps the first two parts were filmed better, otherwise how did it come to the filming of the third movie....
The plot tells us about a girl whose father is being held captive by a man who collects all sorts of macabre things. He threatens to kill her father if she does not bring him a mysterious box in which a demon is imprisoned. To do this, he puts her in a closed boarding school for girls, where the very box is located.
All in all, there is no point in telling the plot further and without fear of making a spoiler. At least a somewhat sophisticated (and not so sophisticated) viewer in the genre, will guess that the box will be opened and the demon will kill everyone indiscriminately.
The weakness and formulaic nature of the plot goes along with the inarticulate character reveals. In addition to the main heroine, there are a few other slightly isolated characters that are also formulaic for such a movie: the local 'school queen' who oppresses everyone and everyone hates her, and the local 'nerd girl'. The only male character is the son of the school principal, who obviously found himself in a closed female boarding school not without the help of the otherworldly (just kidding).
The school in general looks like an ordinary setting, not elaborated at all in the context of the plot. For example, we are not shown the presence of the school's teachers. It looks as if the principal teaches them there herself. In general, except for the principal, her son and the girls, there is no one in the school. The school itself looks like an ordinary old house, or like a castle with large masonry. For unknown reasons there is no electric lighting and everyone walks by candlelight....
The demon itself turned out to be a C-minus. It's not a well-makeup actor, who generally does nothing scary. He doesn't even look scary. In the story, the demon doesn't do any mystical things, he just goes (runs) after girls, aka a maniac from a slasher movie.
Because we don't know anything about the heroines, they don't evoke any empathy or fear for their lives. We are shown indirectly that they have a strained relationship there and hate each other. Guessing who the traitor is is not difficult either. The whole storyline is primitive and there are no 'sudden' endings.