I just spent the whole movie covering my eyes, opening them right before what I knew was going to happen happened ... and then shutting them again and curling up. These movies, man. They get me every time.
You don't go to horror movies for story. You go for sensation, to be moved. Paranormal Activity 3 not only gets that, it also asks you to get it, to be aware of how you're being moved, and your part in the moving.