If you've filled your plate three times, you must have a strong appetite. I hope you still have room for dessert.
The Latin word appetitus, meaning "desire for," is the root for appetite. If you have worked up an appetite, you have a desire for food. If you've lost your appetite, you're not hungry at all (maybe because something upset you). You can have an appetite for things other than food. Perhaps you have an appetite for power, or funny movies, or for little silver shoes with buttons on them.