How do we know? It's all thanks to a custom Sega Genesis cartridge. IGN received a Sonic the Hedgehog 3 film mailer this week that includes a Sega Genesis cartridge that's actually playable.
Those kits include a working Sega Genesis cartridge, complete with custom box art. The cartridge has a minigame featuring Sonic and Shadow, along with character art and posters in pixelated glory.