Combine white Cheddar cheese, Swiss cheese, Parmesan cheese, and cornstarch together in a bowl; set aside.
Stir Irish stout beer, steak sauce, Worcestershire sauce, garlic powder, hot pepper sauce, and mustard powder together in a fondue pot; set heat under fondue pot to medium.
Stir cheese mixture, 1/2 cup at a time, into stout beer mixture in fondue pot until cheese is completely melted and the fondue is thick and smooth. Do not let the fondue boil.
