Legacy serial checks often require administrative privileges to write to the Windows Registry.
Ensure you aren't trying to use a serial from the base game on the Ultimate Collection installer, as they are not cross-compatible. The Modern Way to Play
If you are searching for a to your copy, or trying to navigate the digital DRM of the modern era, this guide will walk you through everything you need to know to get back to building your empire. The Evolution of Settlers 3: From CD-ROM to Digital