What walks on 4 legs at dawn, 2 legs at midday, and 3 legs at dusk?
A man. In the beginning of the day or the begging of his life a baby would walk on it's hands and legs or 4 legs. At midday or the middle of their life they walk on their 2 legs. And at dusk or the end of the day they walk on 2 legs plus a cane which is like a third leg.