Man and Sin by Piet Schoonenberg (1964) 2.1BQ

[Here is another way to say it:

Actuality is always dyadic. God is actual. Both actualities in the dyad must be divine. Both are perceived as Persons.   This accounts for Two Persons.

Dyadic actuality exists in relation to a triadic normal context and a monadic possibility.

God is both actual and relational.

The actuality of “God as a triadic relation” engenders another Person in addition to the dyad.

The triadic relation itself constitutes the third Person.

The Third Relational Person brings the Two Actual Persons into relation with “the Oneness of God”.

The “Oneness of God” cannot be expressed as a Person because it is Pure Potential, that this, Omnipotence.]