The Hebrew word for God is Elohim. The im is plural in Hebrew. El is always used in the singular when referring to the essence of God (reference “Trinity” for additional explanation).
Old Testament references to the Trinity include the pronoun Us where God refers to Himself in the plural (Genesis 1:26a, 27a).
Note: Elohim is used in the Old Testament as a referral to the One True God. But Elohim is also used elsewhere to refer to other “deities” and/or angels, kings, judges, etc. Some anti-Bible types try to use these facts to “prove” that the use of the word Elohim neutralizes the Hebrew use of the word to indicate the One True God. Some even say that because it is used in other ways, we are being “lied to” about God and religion, etc. Their argument disproves nothing.