I may have thought of a slightly different angle on the Christian Trinity. The Jews, including Jesus, Peter and Paul, were, and still are, pure monotheists, like Muslims and Sikhs. Peter and Paul regarded Jesus as a man raised up by God, not as God.
However, the author of the Fourth Gospel deified the Son of God and personified the Spirit of God but remained monotheist. Thus, he had three divine persons but only one divine being. Is this a contradiction?
Can there be three human persons but only one human being? Yes, in schizophrenia, originally called demonic possession: "My name is Legion for we are many..." So the Johannine God might say, "My name is Trinity for We are three." Maybe this is divine self-possession?