Go to Corel's official download center → Legacy products → Enter your serial number to download the original installer. Note that X3 is very old (2006) and may have compatibility issues on Windows 10.
CorelDRAW X3 was released in 2006 for older operating systems like Windows XP and Vista. While some users report being able to run it on Windows 10 using Compatibility Mode Go to Corel's official download center → Legacy