It just like the Scripture says about a husband and his wife: “And the two shall become one flesh.”—Genesis 2.24, Mark 10.8, Ephesians 5.31.
The “one flesh” of matrimony is made up of two persons, a husband and his wife. They are two separate people, two individuals. They can be in two different places at once. One can know more than another at times, the other might take the lead in doing something or even a greater role. But they are always united even when separated by distance.
But in the end it is still one marriage, one union, “one flesh,” at least in the eyes of God. The husband has his own flesh and the wife her own, but they are two fleshes but one.
While one might take the lead, neither is more superior of a human being. They are both equal yet different with different roles. And they are united by love.
Since God is greater than humans, it should make sense that where it takes two humans to make “one flesh,” that God that is greater than us should be composed of three Persons.
Each Person in the Trinity is an individual; they each can be in different places, play completely different roles, one can take the lead or play a greater role, but in the end all three make up one God. They are always united. They are equal even one takes the lead and the others follow. The Father and Son and Holy Spirit are each God, but they are not three Gods but One.
God is a being made up of three individual Persons (Father, Son and Holy Spirit) just as the one flesh of humanity is made up of two individual persons, husband and wife.
ONE GOD = Three Persons.
ONE FLESH or HUMAN UNION OF MATRIMONY = Two Persons.