Cultural choices that seem to draw on roots before historical documents are difficult to explain or to trace to a point of origin. People who spoke Indo-European languages were typically preoccupied with the number three - to have seen things has naturally occurring in groups of threes (many descendant cultures maintain this to this day) or to see their gods as manifesting in three. The old story of Saint Patrick teaching the Irish about the idea of a single god manifested as the Son, the Father, and as the Holy Spirit, using a shamrock as a way to visualize this "difficult" concept, is hard to imagine: the Celts were certainly seeing these relationships long before conversion, and they would likely not have needed this explanation.

Other cultures see things as occurring in fours or in fives, so this is merely a cultural choice that is not easily explained. An excerpt from my Introduction to Folklore may help, especially with your question about "folk stories":

At the turn of the century, Axel Olrik (1864-1917) at the University of Copenhagen took a different approach. This Danish scholar was influential in the early development of Scandinavian folklore. In 1909, he published his “Epische Gesetze der Volksdictung” (“Epic Laws of Folk Narrative”). It was originally presented at a congress of scholars in Berlin in 1908. Other folklorists expanded Olrik’s work based on his notes, publishing new editions of the essay in 1919 and in 1921.

The important contributions of Olrik’s brief article are twofold. First, he broke from a concern about origins and started looking at other questions concerning the nature of oral tradition. His approach anticipates the structuralists after the war, but unlike those later scholars, he does not deny the validity of the Type as a concept.

Olrik’s classic work was made available to the English-speaking world in a translation published in Alan Dundes’s reader, The Study of Folklore (1965: 129-141). Olrik arrived at a series of laws, which he suggested govern oral tradition. These laws would play a critical role in keeping a type hemmed in so that if a storyteller introduced a deviant variation, the next storyteller would correct it.

Rule #2: The law of Repetition: Folk narrative repeats action for emphasis. Olrik pointed out that the hero may go into a field three days in a row to kill giants, or that he tries three times to ride up a glass mountain. In literature, these events would be as different as possible; in folk narrative, they are as similar as possible. Olrik wrote: “every time a striking scene occurs in a narrative, and continuity permits, the scene is repeated.”

Rule #3: 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.