What is the Biblical basis for the "one" in "the Trinitarian God is one being in three persons"? In the Trinity, the one divine being is three persons whereas one human being is only one person (source: Is The Trinity a philosophical contradiction?).

A &quot;being&quot; is an independently existing thing.

The Father, Son and Holy Spirit are not three &quot;beings&quot;.
For instance, Adam is a human being, Eve is a human being and Seth is a human being. You have three human beings because you have three independently existing things.
But the persons of the Trinity are not independently existing things, so we cannot say that there are three divine beings. But rather the Trinity is one being existing as three divine persons.
The point is that they are not three separate things in the way Adam and Eve are three separate beings precisely because they cannot exist apart from each other.
The three do not exist independently. They cannot exist apart from each other. They are one being (literally, one independent thing) and thus, they are one God.
What is the Biblical basis that Trinity is &quot;one being in three persons&quot;?
Did the Scripture say that the Father, Son and Holy Spirit are not three separate beings?
Usually, the Trinity is being defined as of one substance, nature, essence and being. This question specifically asked what exactly does &quot;one being&quot; mean in regards to the Trinity because being is different from nature and person.
My understanding is that the scriptures convey that three Divine Persons share one divine nature.

I and the Father are one. [John 10:30, Young's Literal Translation] . . .

. . . . expresses a shared nature but different personalities.
One could re-word this and say that three Divine Persons share one divine existence.
Fulness being an attribute of divine nature means that the divine nature is shared perfectly, in fulness.

And of His fullness (the Word made flesh) we all have received, and grace for grace.


[John 1:16, KJV]


For in him (Christ) dwelleth all the fulness of the Godhead bodily.


[Colossians 2:9, KJV]

But the word 'being', when it refers to 'human being', has a specific meaning in that human persons, although they share the characteristics of humanity do not actually share the same humanity, that is to say the same human body and the same human soul.
I think that this causes a confusion of wording.
So I therefore, myself, prefer to say that three Divine Persons share one divine existence.
That is to say, three Divine Persons, dwelling in a perfection of unity, co-exist in the fulness of one divine nature.
(This wording therefore expresses person, unity, being and nature.)
And I know that some word this as 'three Divine Persons share one divine essence'.

And in one Lord Jesus Christ, the Son of God, begotten of the Father [the only-begotten; that is, of the essence of the Father, God of God,] Light of Light, very God of very God, begotten, not made, consubstantial with the Father;


First Council of Nicea 325 - Wikipedia


Then, also :

... who, being in the form of God, thought it not robbery to be equal with God:


[Philippians 2:6 KJV]

Here, existence (being) nature (form) deity (God) and person (he) are seen together.
The Son of God exists (being) in the form or nature of deity. That is how his person exists and that is the form in which he exists : deity.
It is similar to the expression 'I am that I am' Exodus 3:14, which expresses the  fact of existence (of being) - I am - and the mode of being or the form of being or the nature of being - that I am.
This existence, this form, he shares (equal with God) : it is an equivalent, an equal, mode of existence with God.
This form of being is a shared form of being - the form or nature in which he exists and which existence he shares : is deity.
It is one form of existence which is shared.
One mode of being - is shared.