Assuming you are referring to a 3lb corned beef brisket, this will feed approximately 6-8 people.
Corned beef is a great option for feeding a large group of people because it is relatively inexpensive and easy to prepare. When cooking a corned beef brisket, be sure to allow for at least 1 hour of cooking time per pound. So, for a 3lb brisket, you should plan on cooking it for at least 3 hours.
Corned beef can be cooked in a variety of ways, but the most common method is to simmer it in water on the stovetop. To do this, simply place the corned beef in a large pot or Dutch oven and cover it with water. Bring the water to a boil, then reduce the heat and let the corned beef simmer until it is cooked through.
Once your corned beef is cooked, you can slice it or shred it and serve it with sides like potatoes, carrots, cabbage, and rye bread. Enjoy!