It might be easiest to think of these three stages in the following way:
marketing - the overall plan
Marketing is how you're going to make sure that you continue to meet the needs of your customers - while increasing your company's value at the same time. A good marketing strategy will highlight ways to increase the future customer base and overview the entire production-distribution systems.
advertising - getting the word out
Advertising uses the strategies from the marketing plan to increase awareness of a product. Advertising itself can come in many forms - tv/radio/print, sponsorship, promotion etc.
sales - closing the deal
Sales tends to be a short(er) term process than marketing. Sales are where new clients are confirmed and the product is sold on a (usually) one-to-one basis.
These three areas of interest are linked in many ways, but should not be confused, or ignored, as they are all vital components of a successful business