Answer:
Homo Sapiens Sapiens!! Is the answer.
Morning is when human beings are in the earliest phase of life i.e. infants usually quadruped - crawling on all four limbs , Afternoon can b synonymous with adulthood where we usually are bipedal, walking using 2 legs and Evening is that phase of senescence or elderly where most of the people use cane as a third supporting leg, hence walks on 3 legs.
This riddle has some history : also known to b the Riddle of Sphinx, the sphinx is said to have guarded the entrance to the Greek city of Thebes, and To have asked a riddle to travellers to allow them passage.
She asks the above mentioned famous riddle. She strangled and devoured anyone unable to answer.
Oedipus is said to have solved this.
And there was a second riddle:
There are 2 sisters: one gives birth to the other and she, in turn, gives birth to the first. Who are the two sisters?
Answer is : Day and Night.
Bested at last, the tale continues, the sphinx then devoured herself.
Great Oedipus !!