The number three has deep significance for a broad swath of territory from western Europe east to (but not including) India. This influences oral tradition, and was identified by the great Danish folklorist Axel Olrik (1864-1917) with epic laws. It seems to be very old and is likely prehistoric, being ingrained in the region's culture (but that last observation can only be speculative based on the distribution and the antiquity of historical occurrences). Here is an excerpt from my draft Introduction to Folklore, discussing Olrik's third (appropriately the third law!):

The Law of Three. In Indo-European and Semitic folktales, repetition occurs in threes. This almost has the appearance of being universal, but there are important exceptions. Where there are three repetitions or three brothers in a European folktale, the story in India will feature four, in keeping with the importance of that number in the subcontinent. Similarly, various American Indian cultures feature four or five as the preferred number, and this manifests in its oral tradition. Olrik made the point that literature, with the need for great realism, has fallen away from this rule, leaving oral tradition as the original form of storytelling exhibiting this rigid approach to narrative.