J. J. M. Roberts provides an explantion of Third Isaiah.
J. J. M. Roberts, "Third Isaiah," HarperCollins Study Bible, ed. by Harold Attridge (San Fransisco: HarperCollins Publishers, 2006), 914
The audience addressed by Third Isaiah is once more living in the land of Judah. Isa 66.1 suggests that work had already begun on rebuilding the temple, work completed in the years 520-515 BCE. The initial return from Babylonian exile has already taken place, but this second exodus has not been as glorious as Second Isaiah predicted, and life for the returnees in Judah remains very harsh. In these difficult circumstances there are both economic oppression and, in an attempt to cope with life's problems, a resurgence in the pagan rituals long indigenous in Israel. In response to this situation Third Isaiah announces God's imminent judgment on the oppressors and syncretists. The prophet promises the righteous that God's glorious deliverance of Israel, long promised by Second Isaiah, is soon to be realized. Soon the wealth of the nations will pour into Jerusalem along with the rest of Israel's exiles, and the shame and sorrow of the recent past will be replaced with eternal joy and prosperity.