You don't need a formal education in rhetoric to recognise the value of the rule of three and use it for yourself. Some folk singers may have simply picked it up by encountering it in the older songs they know and adopted it from there.

Another interesting use of threes comes in the folk canon's more sinister songs. It was once thought that the Devil knocked three times on the door when he arrived to claim your soul - this being his way of mocking the Holy Trinity - and this led to the number often arising in demonology. When The Gosport Tragedy says of Molly's revenge that she "ripped him and stripped him and tore him in three", that may have delivered an extra little shiver to the song's original listeners.