Sayyiduna al-Sa'ib bin Yazid (may Allah be pleased with him) said that the third adhan was that of Friday; Sayyiduna Uthman (may Allah be pleased with him) ordered it when the population of Madinah grew, and (before that) the adhan on Friday was given when the imam sat on the minbar.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2017