From Sahih Muslim (Hadith 3675):
Abu al-Sahba' said to Ibn 'Abbas:Enlighten us with your information whether the three divorces (pronounced at one and the same time) were not treated as one during the lifetime of Allah's Messenger (ﷺ) and Abu Bakr. He said: It was in fact so, but when during the caliphate of 'Umar (Allah be pleased with him) people began to pronounce divorce frequently, he allowed them to do so (to treat pronouncements of three divorces in a single breath as one)
Reference: Muslim 3675