Combine white chocolate and peanut buter in top of a double boiler; bring water to a boil. Reduce heat to low, and cook until chocolate and peanut butter melt, stirring constantly. Spread mixture onto a waxed paper-lined 15x10x1-inch jellyroll pan. Pour semi-sweet chocolate over peanut butter mixture, and swirl through with a knife. Chill until firm.
Cut into 1-1/2x1-inch pieces. Store in refrigerator.
