Isa 48:12-16
12 "Listen to me, O Jacob, even Israel whom I called; I am He, I am the first, I am also the last.
13 "Surely My hand founded the earth, and My right hand spread out the heavens; when I call to them, they stand together.
14 "Assemble, all of you, and listen! Who among them has declared these things? The LORD loves him; he shall carry out His good pleasure on Babylon, and His arm shall be against the Chaldeans.
15 "I, even I, have spoken; indeed I have called him, I have brought him, and He will make his ways successful.
16 "Come near to me, listen to this: from the first I have not spoken in secret, from the time it took place, I was there. And now the Lord GOD has sent me, and His Spirit."

          Notice verse 12 God is the speaker. The one who has called Israel. He states " I am He, I am the first, I am also the last"  Notice "I" & "He" are singular.
Follow the context down and you will see that the speaker who is God and identifies himself as " I am He, I am the first, I am also the last" is also the same one speaking in verse 16. The context does not change speakers. Then this single person (God) speaking says "And now the Lord GOD has sent me, and His Spirit."  In this text God is speaking then he says the Lord God has sent me. Not only has he sent him but he also says "and His Spirit"  What we have here is 3 separate persons in a singular God.

           We find unity in the New Testament in the Trinity as well.
Matt 3:16-17 And after being baptized, Jesus went up immediately from the water; and behold, the heavens were opened, and he saw the Spirit of God descending as a dove, {and} coming upon Him, and behold, a voice out of the heavens, saying, "This is My beloved Son, in whom I am well-pleased."

Matt 28:19"Go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit,

          In these passages we can see all 3, Father, Son & Holy Spirit are separate and also in unity. We can see the Trinity in the birth of Christ as well.

Matt 1:20 But when he had considered this, behold, an angel of the Lord appeared to him in a dream, saying, "Joseph, son of David, do not be afraid to take Mary as your wife; for that which has been conceived in her is of the Holy Spirit.

Matt 16:16-17 And Simon Peter answered and said, "Thou art the Christ, the Son of the living God." And Jesus answered and said to him, "Blessed are you, Simon Barjona, because flesh and blood did not reveal this to you, but My Father who is in heaven.

Mark 1:1 The beginning of the gospel of Jesus Christ, the Son of God.