"Let us make man in our image." (Gen. 1:26)
Some trintiarians may point to this SINGLE instance at Gen. 1:26 as 'proof' of a trinity. Yet, the bible throughout overwhelmingly identifies God by the singular person pronouns "I," and "Me," and "He," and "Him". Trinitarians themselves commonly do not refer to their own Triune God as "They," and "Them". This would not be consistent if God really were a trinity.
When God said "Let us make man in our image" at Gen. 1:26, Jehovah God was speaking to His Son Jesus, His Master Worker, the first-born of all creation (and, possibly, to the rest of the angels also), who were also made in God's image. (Pr 8:30, 31; also compare John 1:1-3; Col 1:15-17) Also see Did Jesus have a pre-human existence?
Also, if the Trinity is correct and God really is composed of three persons, then why does Man (who was created in God's image) not display any kind of a tri-nature about him whatsoever? Certainly if God possessed such a tri-nature, and such a fundamental tri-nature aspect is conspicuously absent in Man, how then could it be said that Man was made in God's image?
We know, however, that the man created by God to be in God's image and likeness (Adam), the son of God (Luke 3:38), was a single person. He could have easily been created with three personalities. But God expressly made him in his image with one mind, one personality: one person.
Also, an image is "a physical likeness or representation of a person"...NOT the person himself. http://dictionary.reference.com/browse/image
So the fact that Man AND Christ (Col. 1:15; 2 Cor. 4:4) are in God's image tells us (1) that BY THE VERY DEFINITION Christ nor Man cannot possibly be that God, and (2) that God is a single person also (to correspond with a man being in His image).
For more, see
Gen. 1:26 "Let Us Make Man in Our Image"
How is Man Made in God's Image? (Gen. 1:26)