I haven't seen any of these movies, but I want to start with Unbreakable sometime soon... But, it was comforting to see that you also can watch this as a standalone movie.
One frequent thing I have read about this movie is that people complain that it can easily be felt that it wasn't planned this way originally, and thus it has been created a third movie, simply to make money and to fill a gap... I am not sure about it, just read it several places!
Oh for sure if you're going to look for it you'll find the gaps and plot holes of a movie that was somehow turned into a trilogy and not planned from the beginning. I do think he came under pressure after the success and this was more of a studio project he had to get out of the way in order to do what he really wants.
He already stated his writing two new original scripts for concepts he wants to explore