The term 'SELLING FAST' can be broken down into two crossword solutions: HOT (3 letters). The phrase 'selling fast' is used to describe a product or service that is in high demand and is quickly being purchased by customers. This could be due to the product's popularity, its limited availability, or its competitive pricing. In this case, the word HOT is an appropriate descriptor for the phrase, as it implies that the product is highly sought after.