Genesis declares that Elohim is THREE and the THREE are Echad/One
Genesis 1:26 - And Elohim(plural) said "Let Us make man in Our image according to Our Likeness"
Echad(One) agrees with the plurality of Elohim.
Echad includes that a plurality can be as 'One' = Genesis declares this when God made Adam and Eve
And He answered and said to them, “Have you not read that He who made them at the beginning ‘made them male and female,’ and said, ‘For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh’?
So then, they are no longer two but one flesh.
Therefore what God has joined together, let not man separate.” - Matt 19:6