Jump scares are cheap!
I remember watching the first movie and noticing how few jump scares there were. Afterwards I did some research and found the director purposefully left them out even after the studio insisted they be put in. I think the idea was to rely on atmosphere and dread rather than cheap scares.
Unfortunately Insidious 3 is filled with cheap jump scares and after a while it just gets annoying. That's not to say the film doesn't have atmosphere – it does. It's just cheapened by jump scares.
Why is it cheap? Because it's not hard to scare people if you constantly raise the volume to 120db. For instance if I was watching a BBC documentary on polar bears, and suddenly the volume went up 200% with high pitched violins, I would jump out of my seat.
My favourite horror movies, perhaps unsurprisingly, are those that don't rely on this cheap tactic. The Ring, for instance, and the Exorcist.
In short, slow build up of dread and horror > high pitched violin at 120db.