Technically, in the Catholic Church, everyone is a saint. However people like Saint John Paul 2, for instance, are people who have reached a point of holiness to where god has done miracles through them. Curing the blind, water to wine, etc... Saints are by no means Gods, simply EXTREMELY holy people.
Mary is different. Mary was immaculately conceived Mother of God. She is pure and completely free of sin, and so if you pray to her and she sees it right, she can ask Jesus (God) to do it. Jesus will do it if mary asks him to, because if Jesus always does what mary asks, knowing she will not ask something impure.
Now as for the Trinity itself, the Trinity is one God, God is the Trinity. Complex yet simple.