Hm you are right, all those extensions are already in use, so either use .t3d or just leave it as it is.
As I said before TSC is an initialism, not an extension used by microsoft. If you check the page for the extension: http://filext.com/file-extension/TSC
you will see its used by TINA pro, circuit simulator software. Low risk for collision among torque users. If you check the page for .t3d however: http://filext.com/file-extension/T3D
its used by Swift 3D, Topaz 3D, and Unreal Engine. High risk for collision.