Sonic 3 Rsdk [new] 〈Official〉

– Because SEGA never released Sonic 3 source data, every object, ring placement, enemy behavior, and boss attack pattern must be hand-coded or extracted from Mega Drive ROMs and translated into RSDK’s scene format.

Because RSDK is a modern engine, some things become easier: sprite scaling, transparency effects (e.g., for Hydrocity’s water), and multi-channel PCM audio. But recreating the feel of the original’s quirky object interaction (like the barrel in Carnival Night) requires obsessive testing. Sonic 3 Rsdk

Here's a brief overview: