The person who increased the number of Adhans for the Jumua prayers to three was `Uthman bin `Affan and it was when the number of the (Muslim) people of Medina had increased. In the lifetime of the Prophet (ﷺ) there was only one Mu'adh-dhin and the Adhan used to be pronounced only after the Imam had taken his seat (i.e. on the pulpit).
Sahih al-Bukhari Book 11, Hadith 37
← Previous Hadith In the lifetime of the Prophet, Abu Bakr and `Umar, the Adhan for the Jumua prayer used to be pronounced when the Imam sat on the pulpit. But during the Caliphate of `Uthman when the Muslims increased in number, a third Adhan at Az-Zaura' was added. Abu `Abdullah said, "Az-Zaura' is a place in the market of Medina."

Sahih al-Bukhari Book 11, Hadith 36 Next Hadith → I heard Muawiya bin Abi Sufyan (repeating the statements of the Adhan) while he was sitting on the pulpit. When the Mu'adh-dhin pronounced the Adhan saying, "Allahu-Akbar, Allahu Akbar", Muawiya said: "Allah Akbar, Allahu Akbar." And when the Mu'adh-dhin said, "Ash-hadu an la ilaha illal-lah (I testify that none has the right to be worshipped but Allah)", Muawiya said, "And (so do) I". When he said, "Ash-hadu anna Muhammadan Rasulullah" (I testify that Muhammad is Allah's Apostle), Muawiya said, "And (so do) I". When the Adhan was finished, Muawiya said, "O people, when the Mu'adh-dhin pronounced the Adhan I heard Allah's Messenger (ﷺ) on this very pulpit saying what you have just heard me saying".

Sahih al-Bukhari Book 11, Hadith 38