Superior Drummer 3 Core Library is not available as a free standalone product. While the software and its massive 230 GB library are delivered via download through the Toontrack Product Manager
Final Pro Tip: Join the official Toontrack Facebook group. Users occasionally sell off their license transfers for half price via Knobcloud or KVR Marketplace. That is the closest you will ever get to a "free" Core Library.
: Unique, high-quality drum sounds provided for free by Spitfire Audio. specific installation error on a licensed version, or would you like a list of legitimate free drum VSTs
When you purchase the software (typically ranging from around $275 to $399), the "free" part of the download refers to the delivery method via the Toontrack Product Manager.