Can I wear a three-piece suit with a belt?

You can't. Accurately speaking, traditionally, you don't need a belt to see through the formal dress, because most of the early formal dresses are three-piece, and a very important function of the vest is to cover your waist, because it was considered indecent to show your waist at that time, and wearing a belt for a vest would destroy the lines of the vest, so even today, if you wear a three-piece formal dress, you should not use a belt. If you wear a two-piece suit, of course, you can use it, but it is not necessary unless it is a very inappropriate ready-made dress. In fact, even the high-end customized men's trousers, especially the formal ones, are often completely unbuttoned. Instead, this kind of soft belt is used on both sides of the trousers to adjust the tightness.