Nobody knows how many of them there are, but we know they exist. They are sometimes called "three-parent offspring," which sounds sinister. But if you met one, you would never know him from any other kid.
These children owe their existence to a fertility treatment called ooplasmic transfer, which involves taking an egg cell from an infertile woman and injecting it with the gel-like cytoplasm from another woman's egg. For some reason this can reverse infertility, possibly because it also transfers healthy mitochondria. Since these bits of cellular machinery contain DNA, children conceived this way inherit genes from their mother, father, and the donor, hence the "three-parent" tag.