For the first time in several years of practice, I saw a teenager with a third testis normally positioned in the scrotum. He had no pain or bother.
Because some third testicles have been found at the time of surgery to correct an undescended testis, third testicles have been associated with testicular cancer, just like undescended testis are.
However, there is no evidence that normally positioned third testicles have an increased risk of testicular cancer.
That is why my recommendation for the teenager was the same that I give to any teenager: “Check your balls”.