What are some suitable foods used for desserts, appetizers and sauces?

Fruits work well for all three. Fruit can be served fresh or baked into a pastry for dessert, served fresh alone or atop a cracker or slice of bread as an appetizer or cooked to make a sauce. Oranges, lemons, plums, and pears work very well as sauces for meat and fishes. Nuts are equally versatile. They can be baked into pastries, roasted and seasoned to make appetizers, or pureed to make sauces like romesco or pesto.