Imam al-Zuhri (may Allah have mercy on him) narrates from Sa'ib bin Yazid (may Allah be pleased with him) that in the time of the Prophet (peace and blessings be upon him), Abu Bakr (may Allah be pleased with him) and Umar (may Allah be pleased with him), the first call was given when the imam came out, and the second when the prayer was established. In the time of Sayyiduna Uthman (may Allah be pleased with him) the number of people grew, so a third call was begun at the place called al-Zawra', and it has continued until now.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5681
Al-Zuhri narrates from Sa'ib bin Yazid (may Allah be pleased with him) that Sayyiduna Uthman (may Allah be pleased with him) ordered the third call when the people of Madinah had grown in number, and the time of the call had been when the imam sat on the pulpit.
Ibn Najiyah (may Allah have mercy on him) said: It was Sayyiduna Uthman (may Allah be pleased with him) who ordered the third call on Friday; the call used to be given only when the imam sat on the pulpit, and the Prophet (peace and blessings be upon him) had only one caller to prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5682