I heard As-Saib bin Yazid, saying, "In the lifetime of Messenger of Allah ﷺ, and Abu Bakr and ʿUmar, the Adhan for the Jumua prayer used to be pronounced after the Imam had taken his seat on the pulpit. But when the people increased in number during the caliphate of ʿUthman, he introduced a third Adhan (on Friday for the Jumua prayer) and it was pronounced at Az-Zaura' and that new state of affairs remained so in the succeeding years.  

سَمِعْتُ السَّائِبَ بْنَ يَزِيدَ يَقُولُ إِنَّ الأَذَانَ يَوْمَ الْجُمُعَةِ كَانَ أَوَّلُهُ حِينَ يَجْلِسُ الإِمَامُ يَوْمَ الْجُمُعَةِ عَلَى الْمِنْبَرِ فِي عَهْدِ رَسُولِ اللَّهِ ﷺ وَأَبِي بَكْرٍ وَعُمَرَ ؓ فَلَمَّا كَانَ فِي خِلاَفَةِ عُثْمَانَ ؓ وَكَثُرُوا أَمَرَ عُثْمَانُ يَوْمَ الْجُمُعَةِ بِالأَذَانِ الثَّالِثِ فَأُذِّنَ بِهِ عَلَى الزَّوْرَاءِ فَثَبَتَ الأَمْرُ عَلَى ذَلِكَ