You should see 28 Days Later and skip 28 Weeks Later. I haven't seen 28 Days Later in a long time but 28 Weeks Later is garbage and you don't need to watch that one. I haven't seen 28 Years Later yet but the newer movie is more of a standalone from what I understand about the movie. You don't need to watch the other two but you should definitely watch 28 Days Later at some point.