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