So before Trinity was made up, the poor guys reading the bible thought "us" meant multiple gods.
I'm not sure it was ever really clear what the plurality in Genesis meant. In the early Genesis, the we, our, us, certainly is addressed to entities all on the same team. But I don't see much evidence of the "we" stuff outside of the creation story in Genesis.
As for the Trinity. It is wholly made up by people who are not described in the Bible. If I had to guess at the reasons, I would suggest that it was done to address accusations from Jews and others that Christianity was poly-theistic and did not properly worship Yahweh. The doctrine is contradicted by Jesus own words.
Je Suis Charlie
Under a government which imprisons any unjustly, the true place for a just man is also in prison. Thoreau: Civil Disobedience (1846)
If there is no struggle, there is no progress. Those who profess to favor freedom, and deprecate agitation, are men who want crops without plowing up the ground, they want rain without thunder and lightning. Frederick Douglass