The person who increased the number of Adhans for the Jumua prayers to three was ʿUthman bin ʿAffan 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).  

أَنَّ الَّذِي زَادَ التَّأْذِينَ الثَّالِثَ يَوْمَ الْجُمُعَةِ عُثْمَانُ بْنُ عَفَّانَ ؓ حِينَ كَثُرَ أَهْلُ الْمَدِينَةِ وَلَمْ يَكُنْ لِلنَّبِيِّ ﷺ مُؤَذِّنٌ غَيْرَ وَاحِدٍ وَكَانَ التَّأْذِينُ يَوْمَ الْجُمُعَةِ حِينَ يَجْلِسُ الإِمَامُ يَعْنِي عَلَى الْمِنْبَرِ