Three popes? Who was the third?
The Greek Orthodox Patriarch of Alexandria is also known as the "Pope
and Patriarch of Alexandria and All Africa".
I recognized Gregory III of the Melkites present, so that accounts for Antioch. Was there any Patriarch of Jerusalem there? That would mean a reunion of Patriarchs of the five apostolic sees!
The full tile of Melkite patriarchs is "Patriarch of Antioch and All the East, of Alexandria and Jerusalem
of the Melkite Greek Catholic Church". So, yes, His Beatitude Gregory III's presence means there was a Patriarch of Jerusalem present.