The easiest way to understand the trinity is with an analogy. I have done this before, but it is worth repeating. Picture a man name Joe, who is a son to his parents, a husband to his wife and a father to his children. He is one person, since he has only one social security number. However, he has three different hats he wears when interacting with his family. He is more submissive and respectful with his parents in ways he is not with his wife and children. He is intimate with his wife in ways he is not with his parents and children, and he is firm but fair with his children in ways he is not with this parents and wife.
In the Old Testament, the one God used the hat of the Father to communicate with the Jews. In the New Testament the one God used the hat of the Son to communicate with the Christians. The Holy Spirit is a third hat, the works of which, over 2000 years, have yet to be compile into the Modern Testament. People can commune with God, in any or all of these three ways, since all hats are from the substance of the one God.
In the example of Joe, he is a good son, father and husband and all his interactions are a blessing to his family. This binds them all.
We can extend this family analogy of Joe, to him also having brothers and sisters, nieces and nephews, and aunts and uncles; trinity plus trinity. This would be the six essences of God; six days, that connects to the extended family of man; other religions. These are the lost tribes of Israel scattered all over the world. This awareness will come only after the Third Testament is compiled, since the works of the Spirit were much broader than expected and impacts all in various ways. Joe can even have grandchildren; seven.