Paranormal Activity 3 followed the same theme as the other two. This was really the first since it was set in 1988 and really brought light to why everything was happening.You DO NOT need to see the first two to understand what is going on. I believe this was the scariest of the 3 and will keep you on the edge of your chair for most of the movie.
The only issue I had with it is the ending, but I will let you decide for yourself.
I recommend seeing this movie with someone who gets scarred easily as this will make the movie much more enjoyable.