I don’t know that the accounts of Matthew, Mark and Luke are all that different; they seem to indicate that she stood at a distance with a large group of Christ’s followers, though Luke does not mention her by name. John, however, indicates that she stood near the cross with a smaller group of Christ’s followers. Be that as it may, this is only a problem if Mary remained in only one location throughout long (3-hour?) ordeal. If she stood in different locations at different times during the ordeal, the gospel accounts are easily harmonized, with each evangelist simply speaking of her location at different times.