Finding or highlighting words in common between multiple passages

I need to do a reverse search - i.e. I've been given a list of search results (each probably around 3,000 words in length) and I need to find a keyword search phrase that will retu all of the correct results and as few other irrelevant results as possible.

One of the ways I was thinking about doing this was to somehow find words in common between ALL of the search results. For example, I would like to find that result A, result B and result C both have the words "Florida" and "Georgia" in common.

I have no idea where to begin to do this. Does anyone have any suggestions for programs or tools to use? Or perhaps there is a better way to accomplish this. If it's relevant, I have a Mac (and a Windows computer without administrative privileges)