Fifthly, the outstretching of Christ's body, and shedding of the blood, and the fruits of the Passion, are signified by the triple signing of the cross at the words, as many as shall receive the body and blood, may be filled with every blessing, etc.. (faith.org.uk)
Seventhly, the three hours during which He hung upon the cross, that is, from the sixth to the ninth hour, are represented; in signification of which we make once more a triple sign of the cross at the words, Through Him, and with Him, and in Him. (faith.org.uk)
It is said that by the holy sign of the cross one can halt the evil Spirit, so that it can not go on. (religion-online.org)