Mythology
Ah! My Goddess has it's roots in Norse Mythology. From mythology, Verdandi (Belldandy) Urd and Skuld were three sisters called the "Norns", or the Three Fates. These three sisters spun the thread of life for all living things. (This concept is also found in Greek/Roman mythology) The sisters lived near the "Well of Urd" at the base of the tree Yggdrasil. Yggdrasil is the tree of the world and connects the nine worlds found in Norse mythology.

The classifications of the goddess in AMG: past, present and future for Urd, Belldandy, and Skuld respectively, also come from the mythology.