The answer isn't quite as literal as the question. A human goes through major stages of development.
In the morning (as a child) the human crawls on all fours.
In the afternoon (mid-age) the human walks upright on both legs.
In the evening (once old age has taken its effect) the human walks with the assistance of a cane, hence three.
Something along these lines I believe.
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Yes!!!!! Well done Riddle of the Sphinx