In an attempt to find the Trinity in the OT, some Trinitarians appeal to the fact elohiym is a plural nown and to the few Scriptures where plural pronouns are used for God such as:
Genesis 1:26
26 Then God said, "Let Us make man in Our image, according to Our likeness; and let them rule over the fish of the sea and over the birds of the sky and over the cattle and over all the earth, and over every creeping thing that creeps on the earth." NAS
However to assume elohiym indicates a compound unity denies that many (if not all) languages have irregular nouns that can be both singular and plural based on how they are used. For instance a sheep is not a compound unity since one sheep and two sheep use the same word "sheep" to indicate quantity. The same is also true of deer.
Elohiym is used throughout the OT to indicate one god. Following as just a few examples.
Exodus 22:20
20 He that sacrificeth unto any god (elohiym), save unto the LORD only, he shall be utterly destroyed. KJV
Judges 9:27
27 And they went out into the fields, and gathered their vineyards, and trode the grapes, and made merry, and went into the house of their god (elohiym), and did eat and drink, and cursed Abimelech. KJV
Judges 16:23
23 Then the lords of the Philistines gathered them together for to offer a great sacrifice unto Dagon their god (elohiym), and to rejoice: for they said, Our god (elohiym) hath delivered Samson our enemy into our hand.
KJV
What about the plural pronouns? Ryrie is a well respected Trinitarian and he views the plural pronouns as plurals of majesty, which agrees with Judaism. One of the most respected commentaries on the OT is Keil & Delitzsch Commentary on the Old Testament.
Genesis 1:24-31
A likeness to the angels cannot be inferred from Hebrews 2:7, or from Luke 20:36. Just as little ground is there for regarding the plural here and in other passages (Genesis 3:22; 11:7; Isaiah 6:8; 41:22) as reflective, an appeal to self; since the singular is employed in such cases as these, even where God Himself is preparing for any particular work (cf. Genesis 2:18; Psalms 12:5; Isaiah 33:10). No other explanation is left, therefore, than to regard it as pluralis majestatis, -
(from Keil & Delitzsch Commentary on the Old Testament: New Updated Edition, Electronic Database. Copyright (c) 1996 by Hendrickson Publishers, Inc.)
The foundation of God's household is the apostles and the prophets, who were Jews who thought like Jews not Greeks. Therefore when we go back to our Jewish roots we can understand that the use of plural nouns and pronouns for God indicate plurals of majesty not that God is a compound unity.