I have nothing much to say about, topic description says it all. The only thing is that I just would like to recommend it specially to people who are intending to make a hack of the same kind of Sonic 3 & Knuckles: Master Edition, with extremely hard levels. Perhaps it would be a good idea to join both works in a single hack. So, this post is also for suggesting everybody who is working in those kind of projects to take a look at this.
For the moment there are two versions. Levels of the first one are hard, but most of them are nothing compared to the new ones coming from the second version. So, it would be better to start with the first version, called Blue Nightmare and if you see you're good enough on it you might play the second version, Red Nightmare. I uploaded a Blue Nightmare gameplay to Youtube, so you will be able to learn some tricks to complete them correctly and having a perfect bonus, however, there are some levels on Blue Nightmare version where you can't get a continue because there are less than 50 rings; this doesn't happen on Red Nightmare. There is also on my channel the 7th Super Emerald stage of Red Nightmare, but not the others. The rest of the new levels are expected to be solved with your own ingenuity.
Sonic 3 & Knuckles: Blue Nightmare (longplay)
First release, recommended for "beginners":
Sonic 3 & Knuckles: Blue Nightmare
Harder and improved version, with 5 (or perhaps 7, who knows...) new levels:
Sonic 3 & Knuckles: Red Nightmare
Enjoy! More coming soon.