It has been said that trying to understand the Trinity is like trying to bite a wall. Yet the Trinity is one of the basic truths and teachings of Christianity. Obviously our finite minds cannot fathom the depth of the infinite God, but He did reveal Himself to us in a specific way and enlightened our minds to allow us to at least consider the concept involved. Let's chew on it.
We experience and observe that each human person possesses a human nature. "Person" is who we are, "nature" is what we are and the source of what we can do. Further, we accept by faith that the Word is a divine person Who at one time possessed only a divine nature. Again, it's one person, one nature. How the Word could somehow take on a second (a human) nature is outside our life experience and thus beyond our total comprehension, but we rely on the inspired Word of God that it is true (John 1:1,14). Like any other mystery of faith, it is something we can only partially understand, but also something that at least we can conceptualize. Thus far then we acknowledge one person, one nature (that's us) and one person, two natures (Jesus).
The Trinity takes us to a unique level of mystery because by that belief we profess the existence of one divine nature possessed by three distinct divine persons. Note that it's not a math problem, as if one equals three, since the persons are the who, the divine nature is the what. And when we understand that the one divine nature is totally perfect and complete in every way and cannot be shared, divided, or distributed, we conclude that the three divine persons must each possess the entire divine nature. This fact also addresses the false accusation that Christians worship three Gods. Unlike our human natures that differ by our individual abilities, the divine nature that the persons of the Trinity each possess is one and the same, thus we profess only one God.
The difficulty in comprehending God's revelation of Himself as three distinct persons, one divine nature, didn't deter the early Church Fathers from writing about it, nor the Councils of Nicea (325 A.D) and Constantinople (381 A.D) from giving it definition. Even though we may have a veiled understanding of the teaching of the Catholic Church on this most fundamental article of our faith, we acknowledge and profess it each time we recite our creed or make the sign of the cross. A most proper response of the faithful in Christ.