From where did the tradition of erecting of three crosses around a church originate?
The Early Church faithful were extremely careful to protect themselves and often disguised the artistic crosses from the local pagans in order to preserve themselves from persecution. Thus it is understandable that crosses were not erected until the end of the Roman persecutions in 313 A.D.
Nevertheless the faithful signed themselves with the Sign of the Cross.
It is probable, though we have no historical evidence for it, that the primitive Christians used the cross to distinguish one another from the pagans in ordinary social intercourse. The latter called the Christians "cross-worshippers", and ironically added, "id colunt quod merentur", i.e. they worship that which they deserve. The Christian apologists, such as Tertullian (Apol., xvi; Ad. Nationes, xii) and Minucius Felix (Octavius, lx, xii, xxviii), felicitously replied to the pagan taunt by showing that their persecutors themselves adored cruciform objects. Such observations throw light on a peculiar fact of primitive Christian life, i.e. the almost total absence from Christian monuments of the period of persecutions of the plain, unadorned cross (E. Reusens, "Eléments d'archéologie chrétienne" 1st ed., 110). The truculent sarcasms of the heathens prevented the faithful from openly displaying this sign of salvation. When the early Christians did represent the sign of the cross on their monuments, nearly all sepulchral in character, they felt obliged to disguise it in some artistic and symbolical way. One of the oldest of the symbols of the cross is the anchor. - Archæology of the Cross and Crucifix
It was only after the Edict of Milan that the Church was able to breathe and construct churches and basilicas in honour of God.
Although there is no direct historical evidence how the three crosses tradition started, it is plausible that it was influenced by St. Helen’s finding of the True Cross in 326 A.D.
Now legend has it that Helen the mother of Constantine not only found the True Cross on which hung Our Lord Jesus Christ, but also the two other crosses and the instruments of the crucifixion.
According to local tradition, in the 4th century St. Helena was inspired by God to travel to the Holy Land in search of the true cross of Jesus Christ.
It is believed that after Jesus’ death, the cross he was crucified on was hidden in a ditch, covered with stones so that the early Christians would not be able to venerate it. In the years that followed, a pagan shrine was also built upon the same site in honor of the goddess Venus, most likely constructed during the reign of the Emperor Hadrian.
There were a handful of people who knew where the true cross was hidden and had passed down that information among themselves over the centuries. One such person, a man named Judas, felt compelled to tell St. Helena where the relics of the true cross were buried.
The miraculous story is narrated in the Golden Legend.
Then Judas made himself ready and began to dig, and when he came to twenty paces deep he found three crosses and brought them to the queen, and because he knew not which was the cross of our Lord, he laid them in the middle of the city and abode the demonstrance of God; and about the hour of noon there was the corpse of a young man brought to be buried. Judas retained the bier, and laid upon it one of the crosses, and after the second, and when he laid on it the third, anon the body that was dead came again to life.
An alternative version of the story is found in the Roman Breviary.
Marcarius, Bishop of Jerusalem, after offering solemn prayers to God, touched with each of the three a woman who was afflicted with a grievous disease. The two first had no effect, but at the touch of the third she was immediately healed.
Both stories have been passed down over the centuries and highlight the fact that three crosses were found, and in order to identify which one was the true cross, the sick were brought to it. When the sick were healed after touching one specific cross, St. Helena believed she found Jesus’ cross.
Whatever the validity of these stories, the tradition of St. Helena finding the cross reminds us that Jesus’ crucifixion was a real historical event and it is through his cross that our wounds are healed. - How did St. Helen find the true cross of Jesus Christ?
The above legend coupled with the fact that Christ was crucified with two others was enough to have started the tradition of churches with three crosses around them. This also makes sense that the symbolism of being that of Calvary. After all, is not the sacrifice of the mass a renewal of Christ’s Sacrifice on the Cross according to Catholic belief! The symbolism is obvious to a Catholic.