What is the difference between the excretory, the respiratory and the urinary systems?

The differences are parts and other bodily functions.

It can be said that the excretory system is composed of multiple coordinating body systems (circulatory, integumentary, urinary/renal, respiratory and digestive) that excretes or expels bodily fluids or wastes that can no longer be of used.

However, excretory is always associated to urinary system, that is why the two terms are almost similar.

Blood is the main medium in excretion, it follows that bloodstreams carries deoxygenated blood and waste materials that is filtered by nephrons in the kidney, for urination or voiding, and exchange of gas in the lungs, for respiration.

The blood, however, in digestion transfers/transports nutrients absorb by the villi, the excess materials which cannot be digested anymore will be egested in a form of feces. (Ingestion- mouth; digestion- stomach and small intestine; egestion- colon and anus).

The liver filters blood coming from the digestive system to detoxify chemicals and drugs. Take care of your liver!

The skin, due to accumulation of excess salts and other bodily fluids, excretes sweat through the process called perspiration. It is also a cooling technique of the body.