In a typical three-prong dryer plug used in the United States, the grounding wire is the third prong, which is a round or U-shaped prong located at the bottom of the plug. This prong is usually colored green and serves as the ground connection to ensure electrical safety. The other two prongs are the hot wires, with one carrying current and the other serving as the neutral wire. It's important to note that electrical configurations can vary, so if you're unsure, it's best to consult the manufacturer's documentation or a qualified electrician to ensure proper installation and safety.