Adobe still holds the copyright for CS3. While Adobe officially removed CS3 activation servers in 2018 (meaning you cannot legally activate a new copy even if you have a valid serial number), distributing the software for free is . There is no legal "free" version of InDesign CS3 unless you purchased it when it was sold.
If you need InDesign on Windows 10, the most reliable and legal way to use it for free is through a of the modern version. adobe indesign cs3 free download for windows 10 best
Adobe has officially retired the activation servers for Creative Suite 3. This means even if you have a legitimate installer and serial number, the software will likely fail to activate and remain unusable. Security Risks: Adobe still holds the copyright for CS3