From my Catholic Source Book:

"God revealed in three persons: the Father(who is God, but is not the Son or the Holy Spirit), the Son ( i.e. Jesus, who is God, but is not the Father or the Holy Spirit), and the Holy Spirit ( who is God, but is not the Father or the Son). seeMt28.18-20;Jn 14.6ff"

also St Patrick's Shamrock:

"The mysteries of the faith are difficult to teach. Legend has it that St. Patrick helped himself to explain the Trinity to the king's two daughters by using a shamrock. Its three leaflets on one stem represent the three persons in one God. Some say the occasion was his sermon to Laoghaire, the chief of the Irish clans who had come to seize the saint for daring to ignite the Easter fire on the Hill of Slane."

    The chief mysteries of Christianity are:

1)Unity

2)Trinity---------*

3)Incarnation----*

4)Death of our Savior

5)Resurrection

    *---The Trinity and the Incarnation are intertwined mysteries.

Trinity, Original Sin, Incarnation have been called "the three Greater Mysteries".