Well the nature of God is a three person nature. However you are correct this doctrine was not even remotely developed during this period in History,
However the word is not YAWEH, the word is ELOHIM.
Elohim is a plural name for God. Now of course we know there is only one God, however this word Elohim, represents the plural nature of God. So before anybody knew of the Holy Spirit, and our Lord Jesus Christ, all we knew of was God. And God is the author of Genesis, God spoke, and Moses, being moved by the Holy Spirit, wrote the words God spoke to hiim. So this is the way God commanded Moses to write, and God used the word Elohim, which is a plural name for God. Thats about as best as we can get with the text.
However we know now, in this latter day, that God is a three person nature, so as we read the “let US make man in Our image” and like language, we know this is God and Christ who is God and the Holy Spirit who is God as the same God, though this was undeveloped, God in His foresight saw the completion of the bible one day and so began to develop the doctrine from the very start, though it is only very recently in the scheme of history, that this could be understood.
John 1 (King James Version)
John 1
1In the beginning was the Word, and the Word was with God, and the Word was God.
2The same was in the beginning with God.
3All things were made by him; and without him was not any thing made that was made.
So we can compare scripture to scripture and see how God develops His doctrine. It wasn’t until thousands of years later, and this gospel of John was written, that we can compare the scripture and see that the We and Our is actually God and the Word who was God.
Genesis 1:3-5 (King James Version)
3And God said, Let there be light: and there was light.
4And God saw the light, that it was good: and God divided the light from the darkness.
5And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day.
John 1:4-10 (King James Version)
4In him was life; and the life was the light of men.
5And the light shineth in darkness; and the darkness comprehended it not.
6There was a man sent from God, whose name was John.
7The same came for a witness, to bear witness of the Light, that all men through him might believe.
8He was not that Light, but was sent to bear witness of that Light.
9That was the true Light, which lighteth every man that cometh into the world.
10He was in the world, and the world was made by him, and the world knew him not.