For the past three nights I have dreamt that my cat has been injured in multiple different accidents. Now I have three cats and I only keep dreaming about my tortishell (calico) being hurt.

Well, since these dreams are specifically about that one cat, it's possible that you have subconsciously noticed that there is something wrong with your calico.

Another possibility is that your cat represents a quality in yourself or in your environment that you feel is being threatened. Think about the qualities that your calico has that are not shared with your other two cats.