Answer:
One possible answer to this riddle is "a human being". A human being starts out life with four legs (as an infant, when they are typically crawling on all fours), transitions to two legs (as a child and adult, when they are able to walk on two feet), and may use a third leg (such as a cane or walking stick) in old age.
This answer is not the only possible solution to the riddle, and there may be other answers that could also fit the given clues. However, the answer of "a human being" is a reasonable interpretation of the riddle, and it is supported by the sequence of events described in the clues.