Deut 6.4 Hear, O Israel: The LORD our God, the LORD is One [echad].
Genesis 2:24 Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one [echad] flesh.
Echad can refer to the word one and to the word unity, as Gen 2:4 indicates.
We understand that things are defined by their attributes; cats have cat dna attributes, apple trees have a different dna attribute than corn, etc.
The FATHER is a Person with the divine attributes of omniscience, omnipotence and omnipresence, etc.
The Son is a Person with the divine attributes of omniscience, omnipotence and omnipresence, etc.
The Holy Spirit is a Person with the divine attributes of omniscience, omnipotence and omnipresence, etc.
The nature of these Divine attributes is such that they form a perfect GODly UNITY so that while there are three divine persons, there is only ONE GOD. That is, the perfection of the unity of divine attributes makes it proper to refer to this unity as ONE GOD, not three gods...even though each of the Three is a Divine being.