Insidious 3 is said to be a prequel set before the haunting of the Lambert family. It reveals how gifted psychic Elise Rainier reluctantly agrees to use her ability to contact the dead in order to help a teenage girl who has been targeted by a dangerous supernatural entity. However, it didn't present itself as a sequel, it didn't seem to set up much for the other two movies, maybe the fact that she was shown a picture of a young boy who may have been the father in INSIDIOUS 1 and 2.
The movie doesn't offer much in the way of plot, starting as a young girl, Quinn Brenner, searches out psychic, Elise Rainier, to answer some questions about her deceased mother. It is somewhat obvious what happens after she does this.
The one huge problem I had with this movie is that the bad guy, the guy with a breathing problem (as he is called in the movie), is never developed. The audience does not know who he is, why he is in the same apartment building as Quinn, how he got to be the monster he was, and how he talked (or forced) others to do what he wanted them to do (which caused their suffering). Being an underdeveloped character made him not as frightening as he could have been, as the audience does not really know what his motivation is for wanting Quinn, or for harming others. The statement that some spirits were just filled with darkness and wanted others to be in the darkness with them just did not sit well, nor did it explain how or why he chose Quinn.
The other huge problem I had with this movie was the father, Sean Brenner, played by Dermot Mulroney. First he seems to easily fall into believing what his daughter is saying, which seems unrealistic. Then second his character was incredibly dull. Now I have seen Mulroney play good and bad characters, and I don't know if it was the writing that turned him into a dull two sided character, or if it was his acting in this one, but it was not very interesting. I found his character to be dull and flat. I would have liked a little more backstory for the family, so I could have been close to them, or at least cared a little about what happened. I mean there was a little brother who seemed to be gone for most of the movie and I wondered what he could have been doing while his sister was being tortured and his father was preoccupied with her injuries and issues.
So, this was not the best of the INSIDIOUS series. I know they are planning on doing an INSIDIOUS 4, and I hope they take it back to the horror and the suspense of the first two movies, That was what made them interesting, this one just makes them regular old ghost/possession stories.