it depends on your personal opinion, if you didn't like the first two then you might not like the third one. personally i thought they were good, i like to put myself into the perspective of the people in scary movies, makes it better :3
Paranormal Activity 3 wasn't good at all! Before I saw the movie everyone told me it was dumb. I saw it for myself and they were right. Nothing from the commercial shows up on the movie. Its definitely not worth your money.