What is the singular thread that binds myth, legend, and folklore together?

To properly answer this question. Definitions are necessary and these have been taken from Dictionary.com online:

- Myth: "a traditional or legendary story, usually concerning some being or hero or event, with or without a determinable basis of fact or a natural explanation, especially one that is concerned with deities or demigods and explains some practice, rite, or phenomenon of nature".

- Legend: "a non-historical or unverifiable story handed down by tradition from earlier times and popularly accepted as historical".

-Folklore: "the traditional beliefs, legends, customs, etc., of a people; lore of a people".

Both myth and legend are two folklore genres but the thread that binds them all together is that the events, characters or situations depicted in these genres are unverifiable and possibly imaginary in nature that is to say: imagination. Of course that varies with each genre: myths mostly involve gods or demi-gods, while legends usually involve humans that are responsible for epic feats and are more grounded in real historical events. Folklore encompasses both since it is the common culture, knowledge an traditions of a particular group.

The singular thread that binds Myth, Legend, and Folklore together is Culture. No matter how big or small, a group of people can be, there will always be a culture that all of its members shared.  

As it's known, Folklore is a group of customs, handicrafts, jokes, dances, music, oral traditions, among others. That any community/social group shares and feels identified with. So keeping in mind what Folklore means; it's easy to identify that, narrative artifacts such as myths and legends clearly expose characteristics that identify the culture of any group of people.