The first Adhan used to be when the Imam sat on the Minbar on Friday, at the time of the Messenger of Allah ﷺ and Abu Bakr and 'Umar. During the caliphate of 'Uthman, when the number of people increased, 'Uthman commanded that a third Adhan be given on Friday, so that Adhan was given from the top of Az-Zawra, and that is how it remained.
أَنَّ الأَذَانَ كَانَ أَوَّلُ حِينَ يَجْلِسُ الإِمَامُ عَلَى الْمِنْبَرِ يَوْمَ الْجُمُعَةَ فِي عَهْدِ رَسُولِ اللَّهِ ﷺ وَأَبِي بَكْرٍ وَعُمَرَ فَلَمَّا كَانَ فِي خِلاَفَةِ عُثْمَانَ وَكَثُرَ النَّاسُ أَمَرَ عُثْمَانُ يَوْمَ الْجُمُعَةِ بِالأَذَانِ الثَّالِثِ فَأُذِّنَ بِهِ عَلَى الزَّوْرَاءِ فَثَبَتَ الأَمْرُ عَلَى ذَلِكَ