The balcony broke off an 18th-century building Sunday evening in the town south of Naples, ANSA reported.
The victims were standing beneath the balcony, watching a religious procession for St. Cyrus.
Maria Vela, 67, and Aniello Scognamiglio, 65, who were cousins, and Concetta Evangelista, 65,were killed, authorities said.
Two of the injured were reported in serious condition. One was in an intensive care unit at a hospital.