
Sonic now falls asleep if you idle for too long, using unused sprites from the Genesis original.Notably, he no longer reuses Sonic 2 sprites when running sideways inside the giant tree in Angel Island 1. Many of Sonic's sprites have been touched up slightly.The camera now features a vertical easing effect, first seen in Sonic Mania.The Drop Dash ability from Sonic Mania has been added.
The title screen background features an animated water distortion effect, instead of the palette cycling seen in the original. The game is internally referred to as "Project Cobalt", and its description is "Knock Knock it's Sonic 3 and Knuckles", a reference to a lyric from Unknown from M.E., Knuckles' theme from Sonic Adventure. For whatever reason, the Sound Test only goes up to 19, meaning that only the main Zone themes and the two standard Boss themes can be listened to. Playing this in Classic mode gives a 4:3 perspective. New Blue Spheres - New Blue Spheres mode. Losing will bring up a message saying, "Super Emerald retrieval unsuccessful." Playing this in Anniversary mode gives a 16:9 perspective. Blue Spheres - Original Blue Spheres mode. Bonus Stage S - Glowing Spheres bonus stage. Bonus Stage R - Slot Machine bonus stage. Bonus Stage G - Gumball Machine bonus stage. Sky Sanctuary K - Knuckles' act (or, more accurately, just a boss fight). Death Egg B - Giant Eggman Robo boss fight, which was originally The Doomsday 2. Hidden Palace - Now has its own entry instead of being considered Lava Reef 4. Lava Reef B - Lava Reef Zone's boss area, which was originally Lava Reef 3.
Extra/renamed entries compared to the original include:."Deathegg" was corrected to "Death Egg".Added alongside this was new shrunken-down icons for all the game's Zones (as previously the icons from Sonic 2 were reused), as well as new character portraits for the Sound Test: The level select now has a level preview icon, a feature that was previously only present in vanilla Sonic 3.The selection screen is more condensed, including more entries such as the "Gumball" bonus stage and all 5 multiplayer stages.The selected character(s) are displayed at the bottom right rather than indicated by a number below Angel Island.