Monotheistic emphasis of the Old Testament is based on the Shema of Deut 6:4

ELOHIM
  The very first verse in the Bible begins with the word Elohim as God which is plural in usage. The Shema also uses the same word. The use of the plural 'Elohim' for God,  instead of the singular 'El' has remained a problem all through history..

This has been generally explained as a 'plural of majesty' or 'singular of intensity'. In most cultures Kings and Religious authorities do use the plural in denoting themself. This is not to imply that they are a complex beings in themselves, but that they represent many. Kings are representatives of people and Priests are representatives of believers before God. So when they speak on that capacity the plurals are used. But we cannot justify God in using the plural term while revealing himself to his creation. God is not representing the creation nor creatures, unless we support the Advaitic thought.  

The word for God in the singular sense is El which is used most often in describing Gods characteristics or attributes such as El Elyon, El Shaddai etc.

EHAD

The word used in the shema for "one"  again denotes a "composite unity". There are two words for 'one' in Biblical Hebrew:  'Ehad' indicating a composite unity i.e. one made up of many parts,  and "yahidh" indicating a strictly monistic oneness.  Shema however do not use the former indicating plurality within the unity. This is again upheld by checking with other passages where ehad being used. 

Gen 2.24--the man and his wife will be one (ehad) flesh.
Man and women are certainly two persons with two seperate bodies and here they become one (ehad) flesh.

Ex 26:6, 11--the fifty gold clasps are used to hold the curtains together so that the tent would be a unit (ehad).
Here we have several distinct curtains forming a unit of one tent using fifty gold clasp.

2 Samuel 2:25--many soldiers made themselves into 'one group' (ehad)
The army is a unity consisting of many soldiers.

Gen 34:16 --the men of Shechem suggest intermarriage with Jacob's children in order to become 'one(ehad) people'.
Here two communities join together to form one people.

In these contexts the statement of Shema is an assertion of unity of Godhead amidst plurality and not the other way round.

 Then we have usage of "us" when god speaks in the passages Gen 1.26, 3:22, 11.7; Is 6.8.

Gen 1.26: Then God said, "Let us make man in our image, in our likeness,
Notice "Us" enhanced and supported by "Our"   Man is created in God's likeness.  How was this likeness reflected in man? This is stated in
Gen 1:27 So God created man in his own image, in the image of God he created him; male and female he created them.
Male and female persons is the image of God.  Male and Female are two different persons.  God's image is that these two are one.  Only at the fall of man this image was broken when male and female asserted their seperateness and domination of one over the other came into existence.  As Adam represents the completeness of man (Adam means mankind), Adam within himself was both male and female forming the Unity.  It was this unity that was seperated by God at the seperation of Eve.  Only after the seperation was creation possible. It is the plurality within the God head that is the start of all creaton.  Until then God was inactive and simply self-existed.  That is why the Gen 1:1 emphasizes the Elohim God.  And we do see the three persons of Trinity at the creation right at verse 1.
Gen 1:1 In the beginning God created the heavens and the earth. 2 Now the earth was formless and empty, darkness was over the surface of the deep, and the Spirit of God was hovering over the waters.3 And God said, (The Word of God)

 Rabbi's recognized that Gen 1.26 was a support for  the plurality of God. In the Midrash Rabbah on Genesis; Rabbi Samuel bar Nahman in the name of Rabbi Jonathan says that , that at the time when Moses wrote the Torah he was writing a portion of it daily.  But  when he came to this verse which says, "And Elohim said let us make man in our image after our likeness," Moses said, Master of the Universe why do you give an excuse to the sectarians to emphazise plurality within you?    God answered to  Moses, "You write and whoever wants to err let him err."  In other words, Moses was particularly instructed to maintain this plurality form specifically.  If that is the case, who is actually erring from the truth? 

Then when we come to the story of fall:
Gen 3.22: And the LORD God said, "The man has now become like one of us,
The problem here is again the plural use of "us".  To whom is God speaking.  If the plurality was a monistic oneness, there is no one else God could identify as us.  The sentence clearly defines a God class or God community.  Again unless we assume a Gnostic God, we have no  other choice than to fall back to a plural organically united God.  

The same usage is again appear in

Gen 11.7: Come, let us go down and confuse their language

and in

Is 6.8: Then I heard the voice of the Lord saying, "Whom shall I send? And who will go for us?"

Thus the plural name Elohim and the possibility of multiple personality within the class of Elohim are probably implied. God being totally other we cannot know the essence of God or his nature in their absolute. Yet God has graciously revealed himself to us and that is all what we know of God. So for confirmation of Trinity - three persons in one Essence we need to look into the revelation of God in history. This is what we attempt in the next two chapters.