אֲשֶׁ֧ר בֵּרֲכ֛וֹ יְהוָ֥ה צְבָא֖וֹת לֵאמֹ֑ר בָּר֨וּךְ עַמִּ֜י מִצְרַ֗יִם וּמַעֲשֵׂ֤ה יָדַי֙ אַשּׁ֔וּר וְנַחֲלָתִ֖י יִשְׂרָאֵֽל׃ (ס)
for that the LORD of hosts hath blessed him, saying: ‘Blessed be Egypt My people and Assyria the work of My hands, and Israel Mine inheritance.’
Ibn Ezra on Isaiah
Whom the Lord of hosts hath blessed. For the Lord has blessed Israel, therefore he will be a blessing, etc; or He has blessed each of the three.32The question is here, to which word does the pronoun in בֵּרְכוֹ he blessed him refer. The one explanation refers it to Israel; because God has blessed Israel, therefore he can again be a source of blessing to other nations. The second explanation refers it to Israel, Egypt, and Assyria, and explains the singular of the pronoun in the usual way, each of them.
Rashi on Isaiah
Blessed is My people Israel, whom I chose for Myself as a people when they were in Egypt.