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.
Season with black pepper to taste.
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