I’m aware that the U.S. was prepared to drop a 3rd atomic bomb based on the existence of the “demon core”. Where was this going to be dropped in Japan and are there any significant facts surrounding this 3rd bomb?
It was never conclusively determined. There were various ideas out there, including:
One of the other targets on their pre-existing target list (Kokura, Niigata)
Don't drop it immediately, save up a few bombs and drop them at once, or use them tactically to aid in the invasion
Drop it on Tokyo
They didn't get to the point of deciding, though apparently they got quite close — just before hearing about the surrender, Truman appeared to have decided he would authorize another one to be used. But he didn't. (It would still have been a week or so until it was used, after authorization.)
See my article here from last summer that tries to collect everything we know about the "third shot."