What kind of T3 is it ? Full, sub rate ? Clear channel, channelized ??
Check some of these guides:
http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_configuration_guide_chapter09186a008020eced.html
Is there any frame-relay to ATM internetworking involved ?
Check this guide for Frame-relay configurations:
http://www.cisco.com/en/US/products/sw/iosswrel/ps1831/products_configuration_guide_chapter09186a0080087d01.html
If its a clear channel t3 or channelized t3, there will be a serial interface associated with it where you can specify encapsulation as Frame-relay (ietf or cisco) and use the above guide to setup the Main 3660.
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus