If you're getting asked for serial key and it doesn't fix itself, you can create a file "regs.reg" and copy this content into it
This will enter the serial key into your registry and should solve the problem of manually entering the serial key every time
REGEDIT4
[HKEY_LOCAL_MACHINE\SOFTWARE\Bluebyte\Siedler3\1.0\General]
"SingleNumber"="0497-2584-2194-4356-23360497-2584-2194-4356-23360497-2584-2194-4356-23360497-2584-2194-4356-2336"
[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Bluebyte\Siedler3\1.0\General]
"SingleNumber"="0497-2584-2194-4356-23360497-2584-2194-4356-23360497-2584-2194-4356-23360497-2584-2194-4356-2336"

need some help please. same problem as below, everytime i try to play Scenario it asks for serial number??

same here Bro

thanks http://igg-games.com

(Settlers 3)
Successful install, game runs. But whenever I try to choose a game mode it asks for a serial number. Game is not playable...
[Edit] Never mind, it worked itself out somehow :3