It could be that you had Unwrap set to channel 3, then pressed "save settings as default" in the edit window menu? http://download.autodesk.com/us/3dsmax/2012help/files/GUID-A7CC32F5-40BE-4DE5-A364-05E7986D4BA-514.htm There's an INI that saves these settings, the file unwrapuvw.ini in the plugcfg directory. Try deleting it.
"Save Current Settings as Default" should be in the Options menu, in the Edit UVWs window. Look it up in the Help file. To clear channels you should use the Channel Info tool in the Tools menu that is part of the 3dsmax main menu. Again check the Help file for details about how to use it, there are some caveats.