Might Makes Right

If I've got three sources that say the same thing then the statement they make is correct. Not necessarily. If source A, source B, and source C agree--do you know where source B and source C got their information? Did they get it from source A? If so, then you really don't have three sources--you've got two sources that copied from another.

It can't always be done, but you want sources that were created independently of each other.

An obituary, death certificate and tombstone agreeing about the date of birth doesn't mean the information they provide about the date of birth is correct--it could mean they have the same informant.