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.
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