Why three bars?
The top bar is the title-board, which Pontius Pilate ordered to be hung in mockery over Christ's head on the Cross. On this board was inscribed: Jesus of Nazareth, King of the Jews, in Hebrew, Greek and Latin (abbreviated in Greek as INBI or in Latin as INRI).
The middle bar is that on which the Lord's hands were nailed. With outstretched hands the Lord opens His arms to us: "Come to me all who are weary and find life burdensome, and I will refresh you. Take my yoke upon your shoulders and learn from me, for I am gentle and humble of heart. Your souls will find rest, for my yoke is easy and my burden light" (Matthew 11:28-29). Jesus willingly takes upon Himself our sins and offers Himself for us.
The bottom bar is the footrest. (In America we use a slanted bar. In Europe, Greek Catholics use a straight bar in order to distinguish a Greek Catholic cross from an Orthodox one.) The footrest is often likened to a balance of righteousness: "Between two thieves Your Cross did prove to be a balance of righteousness: wherefore one of them was dragged down to Hades by the weight of his blasphemy, whereas the other was lightened of his transgressions "This day you will be with me in Paradise" (Luke 23:43).
Information about the three bar cross provided by St. Thomas the Apostle Byzantine Catholic Church, in Rahway, New Jersey.