Why is the nacl extracted with water three times as opposed to only once?

The NaCl is extracted three times to get maximum amount of it. Each extraction only yeilds about 50% of the NaCl from the water. So, after the tird extraction, you would have recieved around 87.5% of the total amount of NaCl from the water(50%+25%+12.5%). After the third extraction, the amount of salt in the water will only be a small amount.