“Spend your life trying to understand it, and you will lose your mind; but deny it and you will lose your soul.”
Augustine in his work “On the Trinity”

A story has been told that Augustine of Hippo was walking on the beach contemplating the mystery of the Trinity.  Then he saw a boy in front of him who had dug a hole in the sand and was going out to the sea again and again and bringing some water to pour into the hole.
Augustine asked him, “What are you doing?”
“I’m going to pour the entire ocean into this hole.”
“That is impossible, the whole ocean will not fit in the hole you have made” said Augustine.
The boy replied, “And you cannot fit the Trinity in your tiny little brain.”

I accept that my tiny brain, ever so much tinier than St. Augustine’s,  cannot possibly absorb or explain the Trinity–I will not try to put the entire ocean in that small hole.  The many analogies used to help human understanding of the Trinity are dangerously limited in scope:
three candles, one light
vapor, water, ice
shell, yolk, albumin
height, width, depth
apple peel, flesh, core
past, present, future.

It is sufficient for me to know, as expressed by the 19th century Anglican pastor J.C. Ryle:

“It was the whole Trinity, which at the beginning of creation said, ‘Let us make man’. It was the whole Trinity again, which at the beginning of the Gospel seemed to say, ‘Let us save man'”.

All one, equal, harmonious, unchangeable, to our rescue.
“It is not easy to find a name that will suitably express so great an excellence, unless it is better to speak in this way:
the Trinity, one God, of whom are all things, through whom are all things, in whom are all things. 
Thus the Father and the Son and the Holy Spirit, and each of these by Himself, is God,
and at the same time they are all one God;
and each of them by Himself is a complete substance, and yet they are all one substance.

The Father is not the Son nor the Holy Spirit;
the Son is not the Father nor the Holy Spirit;
the Holy Spirit is not the Father nor the Son:
but the Father is only Father,
the Son is only Son,
and the Holy Spirit is only Holy Spirit.

To all three belong the same eternity,
the same unchangeableness, the same majesty, the same power.
In the Father is unity, in the Son equality, in the Holy Spirit the harmony of unity and equality.

And these three attributes are all one because of the Father, all equal because of the Son, and all harmonious because of the Holy Spirit.”
–Augustine of Hippo, On Christian Doctrine, I.V.5.