Jamieson, Fausset and Brown on Isaiah 20:3

3. three years--Isaiah's symbolical action did not continue all this time, but at intervals, to keep it before the people's mind during that period [Rosenmuller]. Rather, join "three years" with "sign," a three years' sign, that is, a sign that a three years' calamity would come on Egypt and Ethiopia [Barnes], (Is 8:18). This is the only instance of a strictly symbolical act performed by Isaiah. With later prophets, as Jeremiah and Ezekiel, such acts were common. In some cases they were performed, not literally, but only in prophetic vision.

wonder--rather, "omen"; conveying a threat as to the future [G. V. Smith].

upon--in reference to, against.