I also tend to assume that Turandot wrote her riddles -- they so strongly reflect her vaunted philosophy of life, that hope is a thing of dreams, and men are weak subjects of their passions. She's almost issuing a philosophical challenge at the same time as her intellectual duel to the death "Go ahead, prove me wrong. See, you can't!". And I can't imagine that anyone other than she would have thought to make herself the answer to her own third riddle, particularly not that riddle. Interestingly enough, it just occurred to me, answering that last riddle (and to some degree all of them) is not just a form of beating her with herself; it also requires the victor to correctly assess who she is and how she sees herself. Some who's imposing their own dreams of and passions for Turandot on her might miss that -- "O ragazzo demente -- Turandot non existe!" The trick is not in finding an absolute answer, but in understanding how she would answer.

Also, no one except her actually has a stake in seeing any of the contestants lose, so one imagines if someone else wrote the riddles they might have been a tad easier.