"Clothing" is the most general of the three terms you mentioned here. It basically refers to items that you wear on your body. A "dress", on the other hand, is a noun that refers to a type of clothing usually worn by women. An "outfit" is a combination of clothing items that you choose based on your style. :)

Clothes are things like shirts, pants, skirts, and such that you put together to make an outfit (a shirt and a skirt, for example). Dresses are types of clothes. However you would say "I am getting dressed" and that means the same as "I am putting on clothes."