This is an old and outdated riddle but the official answer is:
Man... young going up.. old with a cane coming down.
This doesn't make sense,most people answered a young man going up the hill,and then a old man with a cane coming down the hill! Hence the third leg,but a cane is not a leg,it's a walking stick used to support ones inability to walk on two legs.
There are many answers, but I would say a plane when it lands.