To whom did Hashem say so?
Radak: Yeshayah heard this in a Nevu'ah.
Why does it say "Shaloshah Shanim Os u'Mofes Al Egypt v'Al Kush"?
Rashi (from Seder Olam 23): Yeshayah must go naked and barefoot, for a sign that after three years, "Melech Ashur will lead the captives of Egypt [and Kush naked and barefoot]" (4). This teaches that Sancheriv sent Tartan to Ashdod three years before his downfall 1 , for he brought the captives of Egypt and Kush to Yerushalayim when he came to besiege Chizkiyah.
Radak (1): Yeshayah was naked and barefoot for three years. This was a sign that so Egypt and Kush will go.
Malbim: Normally, a Nevu'ah for evil can be nullified via Teshuvah, like we find with Ninveh. However, if Hashem commands the Navi to do an act, the Nevu'ah must come, especially if the act is wondrous to those who hear and see it. Os is a sign and remembrance, and Mofes is strange to those who see it, e.g. the command to Hoshe'a to marry a harlot. There is no need to say like Ibn Ezra (Hoshe'a 1:1) and his colleagues (e.g. Radak here, that it was a mere dream or prophetic vision).
This ignores the time to come from Egypt and Kush to Yerushalayim, for it was very small. Sancheriv made 10 journeys on the day that he came to Yerushalayim (Sanhedrin 94b). (PF)
