Awesome news! Good work, Sega! I gotta tell TYP about this, he'll be thrilled.
Actually, I won't, because this is exactly what they should have included in Sonic's "Ultimate" Genesis Collection, which I already bought. I didn't think Sega would actually do this on VC, though, so for folks that don't already own three copies of Sonic & Knuckles, awesome!
I have to wonder if this is "true" lock-on technology, which will retain your game save from Sonic 3, or if Sega's really just including multiple ROM images in the Sonic & Knuckles download and checking your other purchases to unlock variants....