With Biying Dictionary, you will find the definition of the three words. Hope it will help. port (i think you misspell the word as post.): a place by the sea, or by a river or other waterway, where ships and boats can dock, load, and unload harbor: an area of water near the land where it is safe for boats to stay. A port is a harbor where passengers and goods can be taken on and off quay: a platform that runs along the edge of a port or harbor, where boats are loaded and unloaded You can also search for some pictures to help you get a better understanding of the differences of the three.

A harbour (US spelling: harbor) refers to the general area by the sea where boats and ships leave and arrive. A harbour includes water and a quay, which is the ground next to the sea. On quays, ships and boats embark, disembark, load and unload. Post has numerous meanings but I can't think of any meaning which relates to quays or harbours. Can you tell us the context in which you found this word?

A harbor is a place where ships may shelter from the weather or are stored

BINGO! quay is the most specific area among the three.^^