In a large saucepan, combine the sugar, corn syrup and water. Bring to a boil, stirring constantly, until sugar is dissolved. Cook, without stirring, over medium heat until a candy thermometer reads 300° (hard-crack stage).
Remove from the heat; stir in butter, vanilla and baking soda. Add cashews. Pour into a buttered 15x10x1-in. pan. Cool; break into pieces.