The Third Age was the period of time when the various supernaturals of the Middle Kingdom and mankind lived together.
The Kuei-jin, Hsien and Hengeyokai speak of the Third Age as the Third Age of the Great Cycle, the Age of Legends. It was the mythical prehistory of the Middle Kingdom, when the Wan Xian walked among and ruled humanity as benevolent masters. It was also the age where those shen loyal to the August Personage battled the creatures of Yomi under shaky truces. To the Chi'n Ta, this was the age where mankind awakened to Heaven's designs and first learned to wield magic.
The Third Age ended through the machinations of the Yama Kings, who corrupted the Wan Xian to feed on chi. It also marked the end of mythic tradition and the beginning of history as humans understand it today.