This site contains download links for the macOS port of Sonic 3 A.I.R. For more information about the game, or links to the latest version if you are not running macOS, visit the official website for the game instead.
Before downloading, please make sure you have the Steam version of Sonic 3 & Knuckles installed on your Mac. Despite the message on the Steam Store page warning that the game may be incompatible, it is indeed compatible with macOS 10.15 Catalina. This port is compatible with macOS 10.9 and later, including macOS 12 Monterey and is native for both Intel and Apple Silicon.
Note for Apple Silicon Macs: You will need to switch to "OpenGL - Software" in Graphics options from the Options menu if some screen elements do not appear correctly.