For three years, Isaiah appeared in public as God had instructed him, to portray the condition of the Egyptian and Cushite captives that the Assyrians would take in reprisal for stirring up trouble. A Cushite dynasty was in power in Egypt at this time, which accounts for the prominence of Cush in this prophecy. During those three years, Isaiah"s observers doubtless concluded that his condition represented the fate of the people of Ashdod. At the end of three years, God told Isaiah to explain the significance of his strange behavior. That he had portrayed the Egyptians and Cushites, and not the people of Ashdod, would have shocked the Judeans, because many of them favored relying on Egypt and Cush for protection against Assyria. Isaiah"s prophecy was fulfilled in701 B.C. when the Assyrians defeated Egypt at Eltekeh. Another less likely possibility, I think, is Esarhaddon"s conquest of Egypt in671 B.C. [source][source][source]