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
← Previous Hadith I heard Nazi' saying, "Ibn `Umar, said, 'The Prophet (ﷺ) forbade that a man should make another man to get up to sit in his place' ". I said to Nafi`, 'Is it for Jumua prayer only?' He replied, "For Jumua prayer and any other (prayer)."

Sahih al-Bukhari Book 11, Hadith 35 Next Hadith → 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