My informant questioned me with this riddle one day when we were walking back to our apartment. The answer is that it is man because when we are young (in the morning) we crawl on all fours, then as we grow older we walk on two legs and then when we are old (in the evening) we use a cane to help us walk and that is three legs.
This was The Riddle of the Sphinx, which Oedipus had to answer to pass into Thebes. If the traveler was able to answer the riddle correctly, then the Sphinx would destroy itself. However, until Oedipus had arrived, no one had solved the riddle and the Sphinx had eaten all the previous men before him. Apparently there were a few Sphinx stationed around Thebes, Greece and Egypt and all had riddles to be solved.
I think that humans are fascinated by riddles because they exercise the human mind. Also, this riddle in particular uses metaphor to compare a man’s life as times of the day.