Since the Roman Catholic Church originated the formal Trinity doctrine, I feel that they provide the best summary of it:
"St. Jerome says, in a well-known phrase: "The true profession of the mystery of the Trinity is to own that we do not comprehend it"
"Moreover, our Lord's words, Matthew 11:27, "No one knoweth the Son, but the Father," seem to declare expressly that the plurality of Persons in the Godhead is a truth entirely beyond the scope of any created intellect. The Fathers supply many passages in which the incomprehensibility of the Divine Nature is affirmed."
If it's a mystery which is incomprehensible and entirely beyond the scope of all created intellect......