Your facts are wrong. There never were three popes at once--and never two at once either. But there were times when there were two claimants to the office, and once, in the 14th century, there were three claimants.
The question then is: Who was the real pope? No one at the time thought there could be more than one.
Consider an analogy from American politics. In a close election, one might ask: Who is the real president? Even if the answer remained unclear for a while, we'd all know there can be only one president at a time.