The triune God of Christianity is absolutely unique. Except for offshoots of Christianity, other religions have no god, one god, many gods or everything as god. The Trinity is one God existing in the persons of the Father, Son, and Holy Spirit.
The Bible adequately shows the truth of God in three persons and in one being. This mystery was revealed most fully in the person of Jesus and the writings of the New Testament. Jesus had authority to teach us about this because he was one of the persons. Thus, he was able to tell his disciples what they could understand about the Trinity and promised them another person, the Holy Spirit, would help them by providing further knowledge.
This knowledge of the Trinity has not given us the answer to “How can this be?” I believe it is meant to be this way so that we who believe can stand in awe and unbelievers will simply sound foolish when they claim the Trinity is a human invention. As written above, the only person once on earth who might lay claim to the invention of the reality of the Trinity was one of them.