Though you are right that the Blessed Virgin is not part of the Trinity, the Catholic teaching is that she is regarded with favor because of her role in being the Mother of God (quite literally in the Orthodox and Byzantine tradition she is called the “Theotokos” which means Mother of God.) Because of this, and because nowhere does it mention that there cannot be apparitions from Mary and other saints, it’s pretty much canon teaching that she appeared to the three children at Fatima among other apparitions.