MSphinx

Sphinx says.
"I am the Sphinx. I just hope you don't know the answer, as that annoying Oedipus... I always ask the same question (my programmers did not put more questions for me): 
Which is the creature that on the morning walks with four members, on the afternoon does it with two and on the night does it with three?
"Hint: Let's see if you are similar to Oedipus... hehe. It is easy, just three letters"