Is there any scientific evidence that a human has ever grown a third set of teeth?
There are legitimate case reports in credited journals of hyperdontia, or the condition of having supernumerary teeth. Such cases are often associated with congenital syndromes-- cleft lip and palate, trichorhinophalangeal syndrome, cleidocranial dysplasia, and Gardner's syndrome. I included a case report and a comprehensive review for you below. 

Case Report from American Journal of Orthodontic Dentofacial Orthopedics, 2011.

Comprehensive Review from Journal of Oral Science, 2014.