Cersei's prophecy says three children. She's already had three. She won't have any more.
Cersei's prophecy also says a younger, more beautiful queen will take her place. We've all speculated that with the recent revelations of Jon's lineage and legitimacy, it might in fact be Jon Snow who will sit on the Iron Throne. But did Cersei's prophecy tell us who the Ruler of the Seven Kingdoms will be all along?