The first hint of The Trinity appears in the very first verse of the Bible  . Here is an interesting piece of history, the original scribes did not know what exactly Aleph -Tau meant ( the first and last letters of the Hebrew alphabet ) so they just left it as is untranslated. Centuries later Jesus Christ declared he was the Alpha and the Omega ( the first and last letters of the greek alphabet ) and that he was the first and the last. It was then that it finally dawned that He was The Word of Genesis 1:1
Genesis 1 : 1
בְּרֵאשִׁית בָּרָא אֱלֹהִים אֵת הַשָּׁמַיִם וְאֵת הָאָרֶץ
Bereshit bara Elohim אֵת hashamayim ve’et ha’aretz.
In the beginning God Alpeh – Tau created the heavens and the earth.
So now lets look at Genesis 1: 1-2
In the beginning God created the heaven and the earth. And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.
Its as clear as daylight, Father, Son and Holy Spirit in the very first verses of the Bible. Here is another one
“And God [Elohim] said, Let us make man in our image, after our likeness…. (Genesis 1:26)
The verb “said” in the above verse is a masculine singular, but yet the verb “Elohim” is plural. This is one example, there are many other of this type of noun-verb number disagreement would include Genesis 3:22, Genesis 11:7, Joshua 24:19, etc.
“Also I heard the voice of the Lord, saying, Whom shall I send, and who will go for us….” (Isaiah 6:8)
Here, the Lord is specifically said to be speaking, and in mid-sentence He changes His self-reference from singular to plural. In this passage, the Lord appears to be seeking to emphasize His triune nature.
This is one of the most important and revered prayers in the Torah, in fact its one of only two prayers that are specifically commanded in Torah. It is the oldest fixed daily prayer in Judaism, recited morning and night since ancient times  .
שְׁמַע יִשְׂרָאֵל יהוה אֱלֹהֵינוּ יהוה אֶחָד
Sh’ma Yisra’eil Adonai Eloheinu Adonai echad.
Hear, Israel, the Lord is our God, the Lord is One.
The trinity of God is suggested, however, by the use of “echad” in the Sh’ma, which is a word for “one” which is indicative of compounded unity, rather than absolute, monolithic oneness.
I chose the creation and the Sh’ma because of their prominence in the Jewish life. Also Im sticking to only the OT texts, The Trinity can be proved beyond doubt using the New Testament and in the early church writings/beliefs.
There are hundreds of other such examples, I am listing a few below.
Psalm 2: 7-8
I will proclaim the Lord’s decree:
He said to me, “You are my son;
today I have become your father.
and I will make the nations your inheritance,
the ends of the earth your possession.
“Hearken unto me, O Jacob and Israel, my called; I am he; I am the first, I also am the last. Mine hand also hath laid the foundation of the earth, and my right hand hath spanned the heavens: when I call unto them, they stand up together. All ye, assemble yourselves, and hear; which among them hath declared these things? The LORD hath loved him: he will do his pleasure on Babylon, and his arm shall be on the Chaldeans. I, even I, have spoken; yea, I have called him: I have brought him, and he shall make his way prosperous. Come ye near unto me, hear ye this; I have not spoken in secret from the beginning; from the time that it was, there am I: and now the Lord GOD, and his Spirit, hath sent me. Thus saith the LORD, thy Redeemer, the Holy One of Israel; I am the LORD thy God which teacheth thee to profit, which leadeth thee by the way that thou shouldest go.”
“But I will have mercy upon the house of Judah, and will save them by the LORD their God, and will not save them by bow, nor by sword, nor by battle, by horses, nor by horsemen.”
“‘I sent a plague among you after the manner of Egypt; I slew your young men by the sword along with your captured horses, and I made the stench of your camp rise up in your nostrils; yet you have not returned to Me,’ declares the LORD. ‘I overthrew you as God overthrew Sodom and Gomorrah . . . ‘”
Your throne, O God,[a] will last for ever and ever;
a scepter of justice will be the scepter of your kingdom.
You love righteousness and hate wickedness;
therefore God, your God, has set you above your companions
by anointing you with the oil of joy.