You can no longer buy the original standalone ROM directly. Your primary legal option is to purchase Sonic Origins Plus . Why You Might Need the Steam ROM
As of May 20, 2022, SEGA officially the standalone version of Sonic 3 & Knuckles from the Steam store. This was done to encourage players to purchase Sonic Origins , a remastered collection that includes the game alongside Sonic 1, 2, and Sonic CD . sonic 3 and knuckles steam rom download