While the Hebrew word “El” is singular word for God; “Elohim” is a plural word, exercising a concept called “compound unity”. When it is used of the One True God, as opposed to false gods, however, it is normally (although not always) adjoined with a singular verb, such as in Genesis 1:1, “In the beginning, God created the heavens and the earth.” While this by itself does not prove the existence of the Trinity, it does open the door to that concept by suggesting it.
Plural verbs are used along with the word “Elohim” in several places in the Old Testament, such as in Genesis 20:13a, which is typically translated as “…and it came to pass, when God caused me to wander…”. What that text literally reads is “and it came pass when they [in reference to God] caused me to wander”. Also in Genesis 35:7, which is typically rendered as “and he built there an altar and called the place El-beth-el; because there God was revealed unto him”, the Hebrew is actually in plural form that literally reads, “…for there God revealed themselves unto him.” In a third example, 2 Samuel 7:23 is usually interpreted as “…God went to redeem…”, but the Hebrew actually reads “…[For] God they went to redeem…”.

“Elohim” is also applied to two different persons in the same passage in several places. In Psalm 45:6,7, the speaker writes, “Your throne, O God, is forever and ever: a scepter of equity is the scepter of your kingdom. You have loved righteousness and hated wickedness: therefore God, your God, has anointed you with the oil of gladness above your fellows.” Another example is in Hosea 1:7, “But I will have mercy upon the house of Judah, and will save them by Jehovah their God, and will not save them by bow, nor by sword, nor by battle, by horses, nor by horsemen.”

The name YHWH is applied to two different persons in Genesis 19:24, “Then Jehovah rained upon Sodom and Gomorrah brimstone and fire from Jehovah out of heaven”. Another example of this is in Zechariah 2:8,9, “For this says Jehovah of hosts: After glory has he sent me unto the nations which plundered you; for he that touches you touches the apple of his eye. For behold, I will shake my hand over them, and they shall be a spoil to those that served them; and you shall know that Jehovah of hosts has sent me.”

Plural pronouns are used for God in the O.T., such as in Genesis 1;26a, “And God said ‘let us make man in our image, after our likeness:.’.” This could not refer to angels, as man was made in the image of God, not angels. Another case of this is in Genesis 3:22a, “And Jehovah God said ‘Behold, the man is become as one of us.” Yet another example is in Genesis 11:7a, “Come, let us go down, and there confound their language.” Outside of the Pentateuch, we see this in Isaiah 6:8a, “And I heard the voice of the Lord saying ‘whom shall I send, and who will go for us?”.

Examples of plural adjectives are seen in Joshua 24:19, which is typically translated “Holy God”, but in the original Hebrew, due to the plurality of the form for “holy” actually says “Holy Gods”. Similarly Psalm 149:2 literally reads “Let Israel rejoice in his Makers.” Ecclesiastes 12:1 literally reads “Thy Creators”. There are two examples in Isaiah 54:5, which is usually rendered as “Thy Maker is thy Husband”, but which literally reads “Thy makers, thy husbands”.

The Sh’ma in Deuteronomy 6:4 is considered the essence of all forms of Judaism. It says “Hear o Israel, Jehovah our God is one Jehovah:…” Some have used this verse as an argument against the concept of Trinity, but where it is translated as “our God”, it literally reads “our Gods”. The word translated as “one” here is echad, which represents compound unity, as opposed to yachid, which represents absolute unity, and which would have been used if that had been intended. This verse is a great conceptual summary of the Trinity, as one “what” and three “who’s”, unified in substance or essence, yet three individual persons.

In the New Testament, three persons (and no more than three) are called God: the Father, the Son, and the Holy Spirit. Some respective examples are John 6:27, "Do not work for the food which perishes, but for the food which endures to eternal life, which the Son of Man will give to you, for on Him the Father, God, has set His seal." ; John 1:1, "In the beginning was the Word, and the Word was with God, and the Word was God"; and Acts 5:3-9, "Then Peter {said} to her, 'Why is it that you have agreed together to put the Spirit of the Lord to the test? Behold, the feet of those who have buried your husband are at the door, and they will carry you out {as well.}'" .

The unity of these three is taught in Ephesians 6:4-6, "Fathers, do not provoke your children to anger, but bring them up in the discipline and instruction of the Lord. Slaves, be obedient to those who are your masters according to the flesh, with fear and trembling, in the sincerity of your heart, as to Christ; not by way of eyeservice, as men-pleasers, but as slaves of Christ, doing the will of God from the heart."; 1 Timothy 2:5, "For there is one God, {and} one mediator also between God and men, {the} man Christ Jesus"; and James 2:19, "You believe that God is one. You do well; the demons also believe, and shudder". There are numerous additional examples.

Following His resurrection, Jesus makes it perfectly clear who the persons of the Trinity are in Matthew 28:19 with the following command: "Go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit”. Thus, what was somewhat of a mysterious concept in the Old Testament, He has made crystal clear for us in this current Dispensation of Grace.