Badly titled, unfortunately. Unless one assumes these three “ancestors” were fertile with one another, which seems awfully unlikely (robust australopithecines mating with H. erectus?) they can’t all be our ancestors. Stem humans?
It isn’t clear that they could not mate.
Let’s recall that it’s commonly accepted as likely that the chimp and human lineages introgressed to some degree as much as a million years after their separation.
To be fair to the writer, the title is based on a quote from one of the researchers. "there were three very different types of ancient human ancestors roaming the same small landscape,” said Schwartz