If you liked the first two movies you will like the third.
I think one of the most overlooked aspects of Paranormal Activities is the storytelling because it is amazing if you think about it. It's so subtle you can miss all of it and just think of it as a poltergeist movie and that's what makes it so great. The story is basically all over the place, every single scare moment has a purpose. This is not just the typical 'I'M A SCARY GHOST, THIS IS MY HOUSE' thing.
The 'camera work' has reached new heights in PA3 too. Such a little tweak with so much effect.
And yeah, it's scary.
Can't really say which one I liked best.