In a medium saucepan, combine sugar, white corn syrup, and water. Heat over medium heat until boiling, stirring frequently. Add a candy thermometer and stir constantly until mixture reaches about 255 degrees. When you drop the mixture in cold water, it should harden. Quickly beat egg whites in a large bowl and pour the candy mixture in the bowl with the egg whites. Beat vigorously until it is stiff and dull (the mixture should loose its glossy texture. Mix in the vanilla and walnuts.