Genesis 1:26 let us
Genesis 3:22 the man is become as one of us
Genesis 11:7 let us
One of the most common and in my opinion strange proofs for the tradition of the Trinity is found in Genesis 1:26 where God says "let us make man in our image". When we look at this verses you will quickly see that we are missing a lot of information such as who is God speaking to? This is where Trinitarians read into the text and add their own answers. They assume, as I understand it, that God must be speaking to Himself. I believe there is a much more reasonable approach to these verses with some evidence to support it.
At the beginning of Creation God had created the angels. The angels are called the sons of God because they too are made in God's image. I believe that God was speaking to the sons of God that were present at the Creation.
Job 38:
6 Whereupon are the foundations thereof fastened? or who laid the corner stone thereof;
7 When the morning stars sang together, and all the sons of God shouted for joy?
We know that the angels come to us in the appearance of men and they are called the sons of God just as Adam was called the son of God. If the angels are made in God's image, then it makes perfect sense to say to them, let us make man in our image. This isn't to say that the angels were doing the creating, but they were present during the event. We are made in the image of God and the angels are made in the image of God so we are all of the same image.
This works also with Genesis 3:22 where God says "the man is become as one of us". Adam had become as God and the angels in knowing good and evil. We know the angels knew good and evil because some of them have sinned against God. I have wondered if perhaps the tree of knowledge of good and evil wasn't placed in the garden for the angels and not for man. It wasn't just there for temptation, but rather for the angels to eat from. We know that Eve saw that the tree was good for food. How could she see that it was good if she had not witnessed someone eating from it? It is something to think about at the very least.
Again in Genesis 11:7 this would make the same sense as previously noted. God is speaking to the angels who are watching over the affairs of men.
Interestingly in the Book of Jubilees, which is not part of the Bible, God is speaking to the Angel of the presence when he says "It is not good that man should be alone: let us make for him a helpmeet like unto him".