Three (tri) cities (pura), built by thre demons, the sons of Taraka – Tarakaksha, Kamalaksha, Vidyunmali. In some scriptures Kamalaksha is called Viryavana.
When Shiva destroyed it the earth was the chariot, sun and moon the wheels, Brahma the charioteer, Mount Meru the bow, Vishnu the arrow, Agni the arrowhead
Read about Tripura here
http://hindumythologyforgennext.blogspot.com/2011/11/tripurantaka.html