Similar to Demo 2, Sonic the Hedgehog 1995 Demo 3 is another pre-release version of Sonic the Hedgehog from around 1995, offering insight into the game's development. It would likely show further alterations, improvements, or changes made between Demo 2 and the final version.  These demos are important for understanding the iterative design process in creating a classic title like Sonic.

Sonic the Hedgehog 1995 Demo 3

Sonic the Hedgehog 1 @ SAGE 2010