I do also want to align my understanding of the Trinity. This has been haunting me for years. I was explaining Trinity to non-Christian friends when they asked me about the issue. I purposely tried to avoid any theological terms and bible verses as much as I can.
I believe first that there is God (as per old testament), who is full of jealousy, and very strict in teaching the human how to behave. God was like a dictator over human, because back then, human were short-minded and very low in intellectual level. So, like any lower-educated nation these days, the first way to successfully manage the people is though dictatorship.
Soon later, God saw this dictatorship can not work forever. Human mind are designed to improve. So, when the human population increased, God became one of them as Jesus, so He could teach the human about love and humility (New testament).
Later, God saw this “human form” cannot stay for long, because all of his disciples were not naturally smart and brave people. Even after He died on cross, the disciples were in hiding for their lives. So, God became the Spirit that lives in the heart of the disciples. This Spirit then become the living proof that God exists to this day, because, like God, it never dies.
Please correct me if my explanation is not catholic / not correct.