Grease a 9-x13-inch baking pan. In a large, deep saucepan, combine the sugar, water, and corn syrup. Bring the mixture to a boil over high heat, stirring constantly. Stir in the peanuts and continue cooking over high heat, stirring constantly, until the mixture reaches the hard crack stage on a candy thermometer. Remove the saucepan from heat and stir in the baking soda. Pour the mixture into the prepared pan and spread evenly. Cool the brittle completely. When cooled, drop the pan gently on the counter to break the brittle into pieces. The brittle can be stored in an airtight container for about a week.