Not sure where you found the info saying that is the file you need, but its not. The cmputil is for putting the changed files back into the grp files not extracting them.
The file you require is decompress.exe . This can be found here
www.ramius.co.uk/files/decompress.rar
The command to use it (from within a .bat file) would be....
decompress 3D
This will extract ALL the files from the 3d.grp file into a folder called 3D. You would then edit the required file/s and then place them into the main graphics folder to put them back into the 3g.grp file using the cmputil.exe file.
Again using a .bat file, command would be....
cmputil 3D wake.bmp
Filenames ARE case sensitive so be carefull
