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From Sonic Retro

Sonic the Hedgehog 3 Limited Edition, referred to as Sonic 3 + in notes for Sonic Mars, was going to be a compilation of Sonic the Hedgehog 3 and Sonic & Knuckles for the Sega Genesis. This would be similar to what happens when locking the two games together with Sonic & Knuckles Lock-On Technology. It would essentially be Sonic 3 & Knuckles but it would have been contained in a single cartridge and thus probably would have had a cheaper price. It is unknown whether it would have had a port allowing for Lock-On Technology so compatibility with other cartridges is also unknown. If that port were not included then playing as Knuckles in Sonic the Hedgehog 2 or playing the extending Sonic 3 special stage called Blue Sphere would not have been possible. Ultimately, the game was cancelled.

Two gaming magazines, SEGA Magazin and Famitsu, refer to the game. Sega Magazin mentions that the game would be released in addition to the standard Sonic 3 game. Famitsu mentions that the game would be 24 Mbit. The game was also mentioned in an issue of Mean Machines Sega.

Two prototypes of this game were released by drx on February 23, 2008. The earliest can be found here and the later build can be found here. Judging from the protos the game was meant to include an extended version of the Sonic & Knuckles ending theme with music from Sonic 3 mixed in. It also may have used Sonic 3 boss music exclusively.

Sonic 3+ mentioned in Sonic Mars script.