Without looking up the exact definitions, and just going from my intuition, I would say: Apparel is most often used to talk about clothing in a store or when selling clothes. For example, "This store sells children's apparel" Attire is often used to talk about clothing in different settings or seasons. For example, "You should wear semi-formal business attire to the meeting" Garment is most often used when talking about the type of fabric or cloth used to make clothing. For example, "He gave me an expensive silk garment as a gift"