The subject of "The Trinity of God" has been a very difficult subject for many over the centuries. The question is usually raised "How can three (The Father, the Son, and the Holy Spirit) be one (God)? It has even been said "You can't (fully) understand The Trinity, but you must simply believe God's revelation of it in order to truly know God and be saved."
I have never had a problem with understanding the Trinity myself, for some reason, but have met a number of person who have. In order to try to help them understand, as well as those that have been under my ministry as a pastor, I came up with the following illustrations in the white blocks below. They are intended to give an idea of what the Trinity is (One God composed of three persons.) and what the Trinity is not. The Father, the Son, and the Holy Spirit, are NOT all the same person, just in different roles.
Others have tried to use other trinities to compare God to what we are familiar with; using an egg (the shell, the white, and the yolk); water (found as either a solid [ice], liquid [water], gas [condensation]); and even man himself (body, soul, and spirit); but all of these fall short because God is unique. That means there is NOTHING like God to accurately compare Him to. One of the best ways to teach someone something about something that is brand new to them is to compare it to something they are already familiar with. Jesus did this when He used parables. He did this to teach about the kingdom of heaven, and other things, but not of the Trinity of God, for there is nothing to compare to the unique Trinity of God.
At times when we refer to God, we are actually referring to only one member of the Trinity, like when we say "God was nailed to the cross." The Father and Holy Spirit were not nailed to the cross, only Jesus. At other times we are referring to all the members of the Trinity, such as "God created the world, and God indwells the Christian." All three persons of the Trinity were involved in createion. We are not just indwelt by the Holy Spirit, but by all Three (See Mt 10:20; Jn 14:22,23; 17:23,26; 1Cor 6:19,20). When speaking of God in general, God is made up of all Three persons.
The O.T. speaks of man's inability to see God and live; Ex 33:20-23; Dt 5:24-26; Jds 6:22,23. Samson's parents were aware of this, and when they saw an appearance of God, Samson's father thought they were going to die; yet his mother understood that this would not bring their death, but had been a blessing to them; Judges 13:22,23. What the O.T. was speaking of first, was that no mortal man was able to see God IN ALL HIS FULLNESS, and live. That would be like trying to stare into the sun for an extended period of time and not go blind. When God has appeared to man, and they didn't die, it was because God shielded them from seeing ALL HIS FULLNESS. God did this when wrestling with Jacob; Ge 30:32. God did this when Moses wanted to see Him. God did this when people saw the human flesh of Jesus, veiling his glory; Heb 10:20. This glory was somewhat unveiled when Jesus was transfigured upon the mount in Mt 17:1,2.
In the diagram below, the figure of the triangle with bars connecting it to the inner circle is meant to be seen as one continuous piece. Again, this is to represent the Trinity of God as One, made up of three persons that are all equally God, yet destinct personally.