The 3 synoptic Gospels all tell us that “as they led [Jesus] out [to be crucified] they compelled Simon of Cyrene to carry the cross…”. John’s Gospel tells us that Jesus carried his own cross and says nothing about Simon of Cyrene. The tradition is (note it is a small “t” tradition or legend) that Jesus began to carry his cross, but couldn’t go on because of his weakened condition after his brutal flogging. The Romans would see he needed someone else to carry it, but probably only after he had fallen with it. After all, they wouldn’t care if he was having trouble but only that he was holding things up. Someone else might have a quote from an early Church document for this tradition. In any case, it does not violate Scripture to believe he fell 3 times. The fact is, he probably fell more times than that, but 3 is a number that stands for “more than 2 times” in biblical understanding. If the tradition does not violate Scripture then there is no reason why we cannot suppose Jesus fell at least 3 times carrying his cross.