In this post I will give an philosophical argument for there being multiple persons in one God. The argument is as follows:
P1: There is one God who is the greatest possible being
P2: Greatest possible being has every moral perfection to the highest possible degree
P3: Love is a moral perfection
P4: Love to the highest possible degree is not solely centered on ones own person but on another person in love (GL)
P5: God either has GL by his very substance or by things outside of his substance
P6: It is false that God has GL outside of his substance
P7: God has GL in his substance entails that the substance has multiple persons
P8: God has GL in his substance
C: Hence, God is multiple persons
The greatest possible being is such that there can only be one because it is better that God's greatness has no equal rather than there be an competing equal to God. It also seems that the greatest possible being would be a being worthy of worship and hence God is the greatest possible being.
Love to the highest possible degree is not solely centered on ones own person but on another person in love. This seem plausible because when a person only loves himself this seems like not the best kind of love but rather it seems self-centered. For this reason a love that is not for another person is a imperfect love and not a property that is had by the greatest possible being.
This love cannot depend on creation (which is something outside of God) because creation happens to exist by an act of God's free will to create. There are some possible worlds where God does not create and in those worlds God would still be loving even if there are no other created beings. In other words: God's love is necessary and essential, but creation is contingent so created persons cannot be the object of God's love because God's loving nature is necessary.
This argument shows that God is one substance, but this one substance has multiple persons so that the greatest possible being can have the greatest possible love which would be one person loving at least one other person. General revelation using reason as it's instrument can demonstrate such a truth as there being a plurality of persons in one substance but special revelation alone can give us the most precious truth that God is three persons: Father, Son, and Holy Spirit.