Hello, Jayson:
You should be able to do that with the instructions I gave. When it you select your Field in the template window, you can select whatever field you like… including Custom 3.
If you can’t see Custom 3 in your records even after making that change, you may need to edit your journal reference type to show Custom 3. Instructions on modifying reference types is found on page 392 of the EndNote X1.0.1 manual for Windows.