Butter the sides of a heavy 3-quart saucepan. Add sugar, corn syrup, and water; stir to combine. Cook and stir over medium-high heat until the mixture boils. Clip a candy thermometer to side of the pan. Add the peanuts; cook over medium-low heat, stirring occasionally, until thermometer registers 280 degree F to 285 degree F (soft-crack stage; about 45 minutes).