What are the three things on an Altar in a Catholic Church?

Roman Catholic Answer

Outside of Mass, nothing is on an altar but a linen cloth and a dust cloth, perhaps. For Mass, there must be a white linen cloth. There should be a Crucifix and candles NEAR the altar, and on some they may be on it, but not necessarily. At the Offertory, during the middle of Mass, a corporal is put on the altar to catch any crumbs that may fall from the Host. A Sacramentary or Missal will be placed to the left of the corporal. Then the priest takes the paten with the Host, offers it to God and places the Paten on the altar. He does the same with the Chalice. So for the rest of Mass there will be a Missal or Sacramentary (the book), and a Paten and Chalice resting on a Corporal, on the altar. These will all be cleared off after Holy Communion.