You will need to follow these instructions exactly in order to use this fix.
Download the new common.gs script from the link on the right (Right-click and 'save link as' or 'Save target as')
Copy the updated common.gs file into the scripts folder (replacing the current one)
Edit the trainzclassicoptions.txt file and add "-script" (without the quotes) onto the bottom of the file
Start TC3 (It will recompile the scripts so will take a few seconds longer than usual)
Check you can change the start amount of an industry
Close TC3 and edit the trainzclassicoptions.txt file to remove the "-script" line