Just some additional stuff I forgot to put into the check list:
Copy your whole SHIII installation folder and rename it to what you like. Name it GWX or NYGM or what ever you like. The SHIII.exe does not care in which folder it resides.
Take care not to mix up your careers. Dont use a NYGM career for GWX.
You will also need to download Multi SHIII. It patches the hardcoded Name for the folder in OwnDocuments(EigeneDateien)\SHIII\ to another 3letter name of your choice, for instance GWX NYG ABZ XYZ
You can Find the DL Link for MultiSHIII here. Just search for MULTISHIII.
http://www.9teuflottille.de/phpBB2/viewtopic.php?t=6003