Evidence for the Trinity in the Old Testament

It is typical for many groups to claim without a shred of evidence that the Trinity is some kind of modern invention with no basis in the Gospel, and even less in the Old Testament. It is of course Jews that make this accusation more frequently, but Muslims, atheists, and non-trinitarian heretics also love to do the same.

To quickly deal with these groups, this post contains a compilation of pretty much all the evidence you will need to show the presence of three divine persons using only the Old Testament.

Before starting, I will state something that must be clear: while there are many verses saying that there is one single God, that is, that God is one as opposed to being several gods, there is not a single verse stating that the single God is a single person. However, endless verses show the plurality of persons. For the sake of brevity, the content of the verses itself won't be included in this post.

Plural Forms

God is many times referred to in plural form both by Himself and by others (such as Abraham). This is usually answered with the claim that those are instances of "majestic plural", which is ridiculous. The fictional "majestic plural" would be an anachronism, not mentioned by any of the early sources written by Jews hostile to Christianity, such as the Talmud. There is no evidence supporting any "majestic plural".

Furthermore, the earliest witnesses of the Jewish arguments to explain these mention how Jews just claimed that the plural forms refer to God and the celestial courts.

The following verses contain mentions of God in plural, many of which are translated as singular:

Genesis 1:26-27
Genesis 3:5-22
Genesis 11:7-9
Genesis 22:13
Genesis 35:7
Joshua 24:19
2 Samuel 7:23
Psalms 58:11
Proverbs 30:3-4 (rather than plural, mentions how God's Son shares an unknowable name, that is, nature)
Job 35:10
Psalms 149:2
Ecclesiastes 12:1
Isaiah 54:5

The Holy Spirit

The following verses show that the Holy Spirit is both divine by virtue of being uncreated creator that gives true Life (against those who claim it is just some kind of force or even angel) and a different person from God the Father, since He is personified.

Divinity

Genesis 1:2
Job 33:4, related to Genesis 2:7
Job 26:13
Job 33:14-15
Isaiah 32:14-15
Psalms 104:29-30
Ezekiel 37:13-14
Micah 3:8

Personification

2 Samuel 2-3
Nehemiah 9:20
Nehemiah 9:30
Isaiah 40:13-14
Isaiah 63:10
Isaiah 63:14
Micah 2:7
Ezekiel 11:1-5

The Angel of the Lord

Throughout the Old Testament there is one angel referred to as "the angel of the Lord" who is clearly divine, though a different person from the Father. Keep in mind that "angel" merely means "messenger". Also, not every mention of an angel of the Lord has to refer to this divine one.

Exodus 3:1-6 The Angel of the Lord appears in the burning bush, but it is described that God called Moses from it.
Genesis 16:7-14 Agar talks to the Angel of the Lord, who promises her offspring. However, it is said that Agar talked to God.
Genesis 28:10-12 Jacob builds a monument for God in Beth-el and makes vows to him because he found Him there. However, in Genesis 31:10-13, the Angel of the Lord says He is the God of Beth-el where he built the monument and made the vows.
Genesis 32:24-30 describes how Jacob saw God face to face, however Hosea 12:2-5 describes how Jacob fought against the Angel, who was the same one He found in Beth-el.
Exodus 23:20-23 describes how God sends an angel who has the authority to forgive sins, and who carries God's name. This angel is the one that would guide the Hebrews. Furthermore, in Judges 2:1-5, it is the Angel of the Lord who claims to have taken the Hebrews out of Egypt.
Judges 13:3-23 the Angel of the Lord appears to a woman. When her husband asks for his name, the Angel answers "why do you ask for my name? It is wonderful", meaning incomprehensible, occult, extraordinary. The word used (פֶ֛לִאי׃ p̄e·li) is the same used in Isaiah 9:6, in the prophecy that describes a divine Messiah that will be called "wonderful".
Isaiah 63:7-11 describes how God saved Israel, but says that the Angel of His Face (that is, the one in His Presence) saved them.

The Word of God

The following verses personify the Word of God:

Genesis 15:1-6
Jeremiah 1:4-12
Zechariah 4:8-9
Wisdom 18:13-16

Apart from this, there is plenty of Jewish writings that, commenting the Old Testament, identify the Word of God as a separate divine person, the Memra (Word) of God: (Targum Onkelos, Genesis 3; Fragmentary Targum, Genesis 3; Targum Neofiti, Genesis 28; Fragmentary Targum, Exodus 3; Targum Pseudo Jonathan, Leviticus 26; Targum Pseudo-Jonathan, Deuteronomy 4; Targum Neofiti, Deuteronomy 26; Targum Pseudo-Jonathan, Genesis 19; Targum Onkelos, Genesis 9; Targum Onkelos, Genesis 20; Targum Pseudo-Jonathan, Exodus 14; Fragmentary Targum, Exodus 20; Targum Pseudo Jonathan, Exodus 25:22; Fragmentary Targum, Numbers 10:35; Targum Pseudo-Jonathan, Deuteronomy 31, etc.

The Divine Messiah

The following verses show the divinity of the announced Messiah:

Psalms 45:1,3,6-7,11,17 describe a king that is God and has an eternal throne, but is also annointed by God. This divine messiah receives worship.
Isaiah 9:6-7 mentions how one of the titles of the Messiah is mighty God (El Gibbor), a title that Isaiah only uses in another instance to refer to God.
Psalms 110 describes a Messiah that sits to the right of God, sharing His authority and glory. This Messiah is lord of David even though He is his descendant, judges people, and is also a priest according to the order of Melchidezek, even though He is from the tribe of Judah.
Micah 5:2 describes a Messiah that exists from eternity.
Jeremiah 24:5-6 describes a Messiah that will be called "Jehovah, our justice".

The Divine Son of Man

The Son of Man described by Daniel in Daniel 7:13-14 receives the same glory and worship as God, is Lord of His eternal Kingdom, and comes in the clouds, as is said of God in Psalms 104:3 and Nahum 1:3.