What happened to the Boeing Y3 aircraft?
The Y3 project is the third iteration within the Boeing Yellowstone project. Y1 was to be a replacement for the 737, but was put on hold after the decision to move forward with the 737-MAX. Y2 resulted in the 787. Y3 is supposed to address the 350-600+ passenger market, replacing the 777. To my knowledge, no prototype of such an aircraft has been built. Emirates was supposed to be in discussions with Boeing about this, but that was back in 2010, and since then I believe they have ordered the new 777-8X and -9X.