Narrated As-Saib bin Yazid: 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).
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا أَبُو نُعَيْمٍ، قَالَ حَدَّثَنَا عَبْدُ الْعَزِيزِ بْنُ أَبِي سَلَمَةَ الْمَاجِشُونُ، عَنِ الزُّهْرِيِّ، عَنِ السَّائِبِ بْنِ يَزِيدَ، أَنَّ الَّذِي، زَادَ التَّأْذِينَ الثَّالِثَ يَوْمَ الْجُمُعَةِ عُثْمَانُ بْنُ عَفَّانَ ـ رضى الله عنه ـ حِينَ كَثُرَ أَهْلُ الْمَدِينَةِ، وَلَمْ يَكُنْ لِلنَّبِيِّ صلى الله عليه وسلم مُؤَذِّنٌ غَيْرَ وَاحِدٍ، وَكَانَ التَّأْذِينُ يَوْمَ الْجُمُعَةِ حِينَ يَجْلِسُ الإِمَامُ، يَعْنِي عَلَى الْمِنْبَرِ.
sunnah.com