VCDS 21.3 refers to a specific release of Ross-Tech’s diagnostic software, launched around March 2021 (version numbers correlate to the year and month — 21 for 2021, 3 for March). Key features of this release included:

with the interface plugged into the USB port to confirm the software recognizes the hardware. Key Features of VCDS 21.3 Full Diagnostics

Ensure the car's ignition is fully turned on. Verify that the status LED light on your cable is illuminated (usually green or blue). Try swapping to a different USB port on your laptop. "Ross-Tech USB Interface Not Found" in Device Manager

If you're looking for content for "VCDS 21.3 loader download," it's likely because you're using a third-party interface (often called a "clone" cable) that requires a specific loader to function with official Ross-Tech software.