During the time of the Prophet ﷺ and Abu Bakr and 'Umar the call to the Friday prayer was first made at the time when the imam was seated on the pulpit (for giving the sermon). When the time of 'Uthman came, and the people became abundant, 'Uthman ordered to make a third call to the Friday prayer. It was made on al-Zaura' (a house in Medina). The rule of action continued to the same effect. (Using translation from Abū Dāʾūd 1087)  

«أَنَّ الْأَذَانَ كَانَ أَوَلًا حِينَ يَجْلِسُ الْإِمَامُ عَلَى الْمِنْبَرِ يَوْمَ الْجُمُعَةِ فِي عَهْدِ رَسُولِ اللهِ ﷺ وَأَبِي بَكْرٍ وَعُمَرَ فَلَمَّا كَانَ خِلَافَةُ عُثْمَانَ وَكَثُرَ النَّاسُ أَمَرَ عُثْمَانُ بْنُ عَفَّانَ يَوْمَ الْجُمُعَةِ بِالْأَذَانِ الثَّالِثِ فَأُذِّنَ بِهِ عَلَى الزَّوْرَاءِ فَثَبَتَ الْأَمْرُ عَلَى ذَلِكَ»  

"Bilal used to call the Adhan when the Messenger of Allah ﷺ sat on the Minbar on Friday, and when he came down he would say the Iqamah. It continued like that during the time of Abu Bakr and 'Umar, may Allah (SWT) be pleased with them." (Using translation from Nasāʾī 1394)  

«كَانَ بِلَالٌ يُؤَذِّنُ إِذَا جَلَسَ رَسُولُ اللهِ ﷺ عَلَى الْمِنْبَرِ يَوْمَ الْجُمُعَةِ فَإِذَا نَزَلَ أَقَامَ ثُمَّ كَانَ كَذَلِكَ فِي زَمَنِ أَبِي بَكْرٍ وَعُمَرَ»  

"The third Adhan was ordered by 'Uthman when the number of people in Al-Madinah increased. The Messenger of Allah ﷺ only had one Adhan, and the Adhan on Friday was when the Imam sat down." (Using translation from Nasāʾī 1393)  

أَخْبَرَهُ قال «إِنَّمَا أَمَرَ بِالتَّأْذِينِ الثَّالِثِ عُثْمَانُ حِينَ كَثُرَ أَهْلُ الْمَدِينَةِ وَلَمْ يَكُنْ لِرَسُولِ اللهِ ﷺ إِلَّا مُؤَذِّنٌ وَاحِدٌ فَكَانَ التَّأْذِينُ يَوْمَ الْجُمُعَةِ حِينَ يَجْلِسُ الْإِمَامُ»