, the community standard has shifted from searching for a specific "RSDK" file to using Sonic 3 A.I.R. Requirement: You must own the original Sonic 3 & Knuckles (usually via the SEGA Genesis Classics on Steam). Download the Sonic 3 A.I.R. client and point it to your legal ROM file.
– The Retro Engine for Sonic 1 and 2 mobile was eventually reverse-engineered, allowing fan games to run on the same framework. This same framework is now being populated with Sonic 3 ’s data. Sonic 3 Rsdk