Sonic 3D in 2D Secret Switch before the boss room in Gene Gadget Zone Act 1 which lets you fight all 5 Mecha Sonic's at once rather than 3 of them randomly.
Thus, I forgot to mention you have to beat the game 3-5 times with different unlockable characters to get the secret switch.
Anyways I’ll catch ya later but in the meantime bye 👋