Which Sonic the Hedgehog game was originally supposed to be packaged with Sonic 3, but was cut in half due to time constraints?

The Sonic & Knuckles game was originally meant to be packaged with Sonic 3 but was split into two separate games due to time constraints. The reason for this decision was to ensure that the release of Sonic 3 happened on schedule, without compromising the quality of the game. Sonic & Knuckles was designed as an attachment that allowed players to connect it to Sonic 3, combining the two games into one, complete experience. This split ultimately allowed both games to be released individually, maintaining the promised release date for Sonic 3 while giving players the option to connect the two games together if they owned both.