A three-piece suit is a suit that consists of a jacket, pants, and a vest. It is typically considered a formal and polished type of attire, and is often worn to special occasions such as weddings, black tie events, and formal business meetings. The vest, also known as a waistcoat, adds an extra layer of warmth and can be a stylish and elegant accessory. Because of its formality, a three-piece suit is not typically worn in more casual settings or everyday work environments. If you are unsure about whether or not a three-piece suit is appropriate for a particular occasion, it's always a good idea to check the dress code and take cues from the other attendees.