The Trinity is a mystery that is dogma and must be believed for one to call themselves a Christian. This is a leap of faith, because though we know it is true, we are not able to understand everything about it. Do we need to understand everything about it in order to believe? Some would say that to believe we must have absolute knowledge of the subject. To not have this knowability is a contradiction in eyes of many.
There are many things that we have knowledge of, but we do not know absolutely. The medical field is constantly changing and filled with new advances, but just a few decades ago the damage of cigarettes on the human body was not well known. Is this a contradiction in the medical field? Do we not adhere to the advice of our doctor because we do not have an absolute knowledge of his field? To have that line of thinking borders on insanity.
There is no tension between the trinity and its knowability. The Trinity was revealed very slowly in scripture because to reveal it right away would lead Israel into Tritheism. They simply would not have understood it. The members of the Trinity were together at one time at the baptism of Christ, and Christ mentioned all three. For those who have issues believing the Trinity, St. Augustine asks a very interesting question. Do you believe Jesus rose from the dead though you have never seen anyone else do the same (Augustine 7.5)? We love the Lord Jesus though we have never seen him, and we love the other members of the Trinity as well. We see the handiwork of the Trinity all around us. The Trinity is one God with three persons, and we love them because they are God. It does take an element of faith like most things in life. That illumination that faith provides assists in understanding it a bit more. If we fully understand everything there is to know about God, then he ceases being God.
Augustine of Hippo. Philip Schaff. (Buffalo, NY: Christian Literature Publishing Co., 1887.) Revised and edited for New Advent by Kevin Knight. <http://www.newadvent.org/fathers/130101.htm>, accessed November 11, 2018.