I have buy and instal the Settlers 3, but when I try to start the game, she ask me about a serial number and when I go on the workshop they say that you can find it on your profil page but I didn't find at this time, someone knows how to get a serial number or where I can find it?

I want to know this too

go in C:\GOG Games\Settlers 3 Ultimate/ and run regs.cmd as Administrator

thanks, that worked!