What a clear passage of the Triune God in the OT: "Approach Me and listen to this. From the beginning I have not spoken in secret; from the time anything existed, I was there. And now the Lord GOD has sent me and His Spirit" (Is. 48.16). "Thus saith the LORD, the Redeemer of Israel, and his Holy One, to him whom man despiseth, to him whom the nation abhorreth, to a servant of rulers, Kings shall see and arise, princes also shall worship, because of the LORD that is faithful, and the Holy One of Israel, and he shall choose thee" (Is. 49.7). Who is the Holy One of God the Father? Jesus. Men despise Jesus even had Him killed. Who is there who is a Christian leader who runs a nation? God has chosen His only begotten son to pay for sins to save people. The only begotten is the One whom is co-equal.
More Old Testament proof Jesus is in the Triune Godhead. "1 Behold my servant, whom I uphold; mine elect, [in whom] my soul delighteth; I have put my spirit upon him: he shall bring forth judgment to the Gentiles. 2 He shall not cry, nor lift up, nor cause his voice to be heard in the street. 3 A bruised reed shall he not break, and the smoking flax shall he not quench: he shall bring forth judgment unto truth. 4 He shall not fail nor be discouraged, till he have set judgment in the earth: and the isles shall wait for his law. 5 Thus saith God the LORD, he that created the heavens, and stretched them out; he that spread forth the earth, and that which cometh out of it; he that giveth breath unto the people upon it, and spirit to them that walk therein: 6 I the LORD have called thee in righteousness, and will hold thine hand, and will keep thee, and give thee for a covenant of the people, for a light of the Gentiles; 7 To open the blind eyes, to bring out the prisoners from the prison, [and] them that sit in darkness out of the prison house" (Is. 42.1-7).