Combine the sugar, corn syrup, and water in a large (5 quart works well) nonstick pot and attach a candy thermometer to the side. Cook over medium heat, stirring constantly, until the sugar dissolves.
Bring the mixture to a boil, stirring frequently, then stir in the butter. Continue cooking until the temperature reaches 230◦, when you'll need to begin stirring constantly again.
When the temperature reaches 280◦, add the nuts and continue stirring constantly.
When the mixture reaches 305◦ (hard crack stage), remove it from the heat, and very quickly but thoroughly stir in the baking soda.
Immediately divide the mixture between two ungreased nonstick baking sheets and very quickly spread into a thin (about 1/4-inch) even layer on each. The mixture should only cover about two-thirds of each pan.
Let the pans sit undisturbed until completely cool. The brittle should then easily release from the pans and can be broken into small pieces.