I think that Insidious might genuinely be a franchise best watched in the story's chronological order, rather than the movies' release dates. I can't be certain though, because I'd have to re-watch them all to make sure I'm not making a bad recommendation, and I'm not super keen on doing that seeing as this one and the one preceding it were both kinda... bad.

The mythology of Insidious is something I am very interested in, and The Last Key added it to it somewhat. But if this is the sort of quality we can expect from future instalments, then maybe we should never see the resolution... lees de rest.