© 2012 B. L. Freeborn (updated Nov. 2018)
What one thing is father, son and ghost to itself?
One.
1 = 1² = Square Root of 1 = 1/1 = 1×1
It is the mathematical identity principle.
And from One we create all….1 + 1 = 2, 1 + 1 + 1 = 3 and all else….
Let us not forget:
“The Source. There is only One. It is indivisible, exists everywhere, and everything is a part of it. The All is part of all, and all are part of the All.” —J. Rankin
noahsage 
Sorry to say, but nobody can explain the trinity. All the apologists do is cite bizarre examples like three states of water or three parts of egg to define the nature of complex incomprehensible God.
My guess is the mystery schools were teaching advanced mathematics. They had a name for everything like square root is “son.” And “father” is a number’s square. (Maybe reverse that.) Elohim, one of the Hebrew names for god, meant something else and Moses meant something else.
If each name means something mathematical, then they comprehended the infinite-incomprehensible in a rather scientific manner.
Since most people, then and now, are ignorant of math, it would remain a mystery.