The New American Standard Bible Updated Edition seems to assume that Isaiah 48:16 is speaking of the Triune God, in that the words Me and Spirit are capitalized. Not so the NET Bible, however, which does not capitalize either word.
The reason for the NET's approach to the verse is that the NET's translators are perhaps thinking of a man, an earthly king known as Cyrus the Great, who would be an instrument in the hand of the Lord God to defeat the Chaldeans/Babylonians who had held Israel in captivity for 70 years.
"Approach me! Listen to this!
From the very first I have not spoken in secret;
when it happens, I am there.”
So now, the sovereign Lord has sent me, accompanied by his spirit."
In Ezra, chapter 1, we read the following:
"In the first year of King Cyrus of Persia, in order to fulfill the Lord’s message spoken through Jeremiah, the Lord stirred the mind [the Hebrew noun רוּחַ, ruakh, or spirit] of King Cyrus of Persia. He disseminated a proclamation throughout his entire kingdom, announcing in a written edict the following: 'Thus says King Cyrus of Persia:
The Lord God of heaven has given me all the kingdoms of the earth. He has instructed me to build a temple for him in Jerusalem, which is in Judah'" (vv.1-2a NET).
Notice who stirred up King Cyrus's mind (or spirit): the LORD. Proverbs 21:1 tells us that
"The king's heart is like channels of water in the hand of the LORD ; He turns it wherever He wishes."
In other words, God's wish is the king's command, so Cyrus declared that the Jewish exiles in Babylon were free to return to Judah to rebuild the House of God in Jerusalem, and Cyrus even provided the financing to make the trip and the building project possible! Moreover, Ezra tells us,
"Then the leaders of Judah and Benjamin, along with the priests and the Levites--all those whose mind God had stirred--got ready to go up in order to build the temple of the Lord in Jerusalem" (1:5).
In conclusion, then, God stirred the spirit of a king and he stirred the mind of his chosen ones, his leaders of the Judahites and Benjamites to accomplish his purposes and plan. Very likely Isaiah 48:16 does not contribute to the teaching of the Scripture on the triune nature of God, but it does tell us that the LORD is sovereignly in charge of the affairs of men, and in particular "his people" (see Ezra 1:3).