In Eastern Orthodox and Eastern Catholic Churches, the thumb, index, and middle finger, brought to a point, symbolizing the Trinity, The Father, Son, and Holy Spirit/Ghost, sharing a Single Essence. The remaining two fingers, kept pressing together, and touching the palm, representing the human and Devine natures of Jesus Christ. So starting with the head and the moving right shoulder to left and then the below the chest, represents the body and blood of Christ, as he was hammered to the cross with nails, to die for our sins.
Do you mean when the make the cross symbol on their heart? I believe it's Father, Son, and Holy Spirit. But... I am not Catholic, so if your question goes any deeper, I am not sure how the Catholic religion answers it.