Before hunting for modified ROMs, consider supporting the official rereleases. Sega has ported Sonic 3 & Knuckles to several modern platforms. While these versions do not include Shadow, they offer the purest experience.
The Ultimate Lifeform in a 16-Bit World: Exploring Shadow in Sonic 3 & Knuckles Shadow In Sonic 3 And Knuckles Rom Download
Happy hacking, and keep your rings close. Before hunting for modified ROMs, consider supporting the
Many "free ROM" sites bundle adware, browser hijackers, or fake .exe files disguised as ROMs. Stick to known archives or rip your own cartridges if you legally own the original game. Before hunting for modified ROMs