Within a developed worldview, there is often a presentation of cosmological concepts. Not to mention an enumeration of worlds. Though the number varies in various Indo-European cultures, three is a very common number. John Shaw, in 'On Indo-European Cosmic Structure' covers this very well. (The link shared is to academia.edu, free to sign up, and …
Custom of the New Galatîs
Bessus Nouiogalation 