Sonic the Hedgehog 3C Prototype 0517 is a prototype for the canceled Sonic the Hedgehog 3 and Sonic & Knuckles compilation called Sonic the Hedgehog 3 Limited Edition for the Sega Mega Drive. It claims to have been built on May 17th, 1994 at 17:08.
This prototype was released by drx during the February 23rd, 2008 proto release. An earlier in-development beta with a compilation number of 0408 was also found. When this beta was obtained by drx the file was labeled "3C" indicating this was an unreleased version of Sonic the Hedgehog 3. Due to the late development date (dating after Sonic 3's release), it's obvious this isn't Sonic 3 before it was split into 2 games.