There is no "three source" rule in genealogy. Just because three sources agree does not mean that they are necessarily correct. They could actually have the same informant giving the same information at different points in time for different records.
The key is to try and obtain the most reliable accurate sources you can and augment that with additional sources and information--ideally with other informants providing the information.
Two independent sources that agree can easily be correct. But there's always the chance that a new source provides conflicting information.
There are eight or more sources that indicate my great-great-grandmother's maiden name was Haase.
Problem is that those sources are all incorrect. Contemporary sources to her birth provide her actual maiden name of Bieger.