There is no "three sources rule."
There is no magic number of sources that something is proven. Might does not make "genealogical right."
Some sources are more reliable than others and some sources are actually derived from others. After all, if your grandma was the informant as to her place of birth on her marriage license and on her social security number application, is that really two different sources for that information? After all, Grandma is one person.
Each source needs to be evaluated individually as to how reliable you perceive it to be. In some cases, one source may be all you need. In other situations, ten sources may not be enough.