It is like mushroom gathering. If you see a few mushrooms, you would look around for more. (It was a favorite family vacation activity when I was a child).
If a web page has (about) three values of the same kind – for example, company names or email addresses – it will likely have more.
There are two ways to exploit this idea.
Research
If you are looking for a product (of any kind) and know some product names, put 3 (or 2-4) of them in a search string to find pages with lists of those products. For example, if you are researching best Chrome Extensions for finding emails, list three you know of in your string to find more. It is the same story for threes of target company names, etc.
Then, if you wish, you can create OR strings out of the values you find.
Sourcing for Lists
Contact lists of professionals that Google can find is a largely untapped resource in recruiting. That is because most pages with lists are not ranked very high. You will not find them by Googling unless you make an effort.
If you search for something like a list of nurse practitioners in florida, your top results will be sites that sell that data. But if you search for threes of first names, company names, job titles, email domains, or phone extensions, along with some “list indicators” like list, directory, or roster, results will be beautiful.
You will find examples of the Rule of Three for list-searching at the end of this post.
In the just-delivered class LinkedIn Solved, I spoke about utilizing both techniques to proceed to find potential candidates on LinkedIn.