None of these Zones are accessible without locking onto Sonic & Knuckles, though the incomplete Flying Battery is accessible via hacking.

The standalone Sonic 3 cart has Mushroom Valley note the original name of Mushroom Hill Zone, Flying Battery, and Sandopolis listed in its level select.

In the sound test, most of the Zone music that exists in track 09 onward is from Sonic & Knuckles.

In fact, all of the Sonic & Knuckles music is in the Sonic 3 cartridge as well.

You can hear it via a glitch during both the Hydrocity Zone Act 1 and Act 2 boss fights if you wait until the drowning music starts, then jump out of the water (a glitch which also works for the Act 2 boss in the combined version).

The standalone Sonic 3 cartridge has the Sonic & Knuckles miniboss music dummied out.

Another reversed example: the infinite oxygen of the Water Shield is useless in the standalone Sonic & Knuckles, as it does not contain a single underwater section.The boss itself appears as a second phase in Launch Base for Knuckles instead (with standard boss music and no darkening of the sky), while the final boss music is repurposed for the opening phase of each of their respective final bosses (second boss of Death Egg Zone Act 2 for Sonic/Tails, Mecha Sonic for Knuckles). A reversed example: in the combined game, when you play Launch Base Zone (the final level of Sonic 3) as Sonic or Tails, the game skips over the original final boss of the level ( including the awesome ending music).Much of Knuckles' content for the original Sonic 3 zones is present in the Sonic 3 cartridge, but can't be accessed without attaching it to the Sonic & Knuckles cartridge.Sonic 3 Limited Edition, a cancelled Updated Re-release of the game, featured an updated credits medley which included the standalone Sonic 3 tracks, alongside the Sonic & Knuckles tracks.

While it's exact purpose is unknown, many in the fandom speculate that it was originally planned to be a unique theme for Super Sonic ala Sonic 2, until it was replaced by the invincibility theme.

Besides the aforementioned tracks, there was another unused theme in the Prototype/ Sonic & Knuckles Collection that never got used at all.

Sonic 3 complete rom