John has an interesting comment about the Trinity.  (I have told you before that the word "trinity" doesn't occur in the Bible.  It is a word we made up to describe something that is very hard to understand.  But John tries to explain it.

1 John 5:7  "For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one."

 Paul explains the Father-Son relationship in the letter he wrote to the Philippians.  He says that God took on the form of a man.  So that we could see him.  So we could get to know him.  To understand what he was like.  To die for us.  To redeem us unto himself.
Philippians 2:5-7  "...Christ Jesus, who, being in the form of God, thought it not robbery to be equal with with God, but made himself of no reputation, and took upon himself the form of a servant,  and was made in the likeness of men..."

So God became a man.  But the hardest part for me is understanding what Jesus was like when He became a Spirit.  I know He returned to us as the Spirit of God--to live in our soul and heart.  But once again--He is invisible.  Of the three forms of God, I understand Jesus the best because he was human like me.  Physical.  Even though He was God as well.

Maybe you could think of this abstractly as you do H2O.  It always has the same composition, but it can appear as ice, liquid, or steam.  And the function changes as the form changes.  Or maybe you might think of it this way:  I am a mother, a wife, a friend, a professor, a Bible teacher, etc., each part of me with a unique role.  The people I interact with do so depending on how they see me.  I am the same person, I just behave differently in each of my roles.

Whatever.  God is God is Jesus is the Holy Spirit.  The Lord our God is one God.