 an additional cart that sat between the cartridge itself  and the console to enable the manipulation of code to gain extras  like more lives or invulnerability). The cartridge has a cartridge  slot on the top which you could plug in any Mega Drive/Genesis  cartridge. Plugging in Sonic 3 allowed you to play Sonic 3 as it was  meant to be, allowing you to play both games combined, as well as  use Knuckles in the Sonic 3 stages (which had different routes than  Sonic & Tails stages), as well as Tails in the Sonic & Knuckles  stages which also allowed Tails to become Super Tails for the first  (and to my knowledge only, excluding the debug cheat in Sonic 2)  time, he had the same abilities as Super Sonic & Knuckles, but he  also had 4 invulnerable birds which would attack any enemy nearby.  You could also become Hyper Sonic & Hyper Knuckles which were  supped