This is an old riddle (really the oldest riddle ever).
The answer is a human.
At the beginning of life (the morning) he crawls as a baby on all fours. During the middle of his life he walks on two legs (noon) and at the end of his life (the night) he walks with a cane (three legs). The cane is an ancient symbol representing knowledge according to Egyptian belief.
This is the classic Riddle of the Sphinx; the Sphinx at Thebes posed the riddle to travelers; if they failed to solve it, they were killed, if they answered it correct the Sphinx would kill itself; Oedipud answered it correctly
It is from Greek Mythology.