Line an 8" square pan with aluminum foil so that it extends at least 2" over sides of pan. Lightly coat pan with cooking spray. Set pan aside.
Combine semisweet chocolate and 1 tablespoon peanut butter in a 1-quart glass bowl. Microwave at HIGH 1 minute. Stir well, and microwave at HIGH 15 seconds to 1 more minute. Stir gently until chocolate melts. Spread chocolate into prepared pan, spreading until smooth with a narrow metal spatula. (Do not clean bowl.) Chill chocolate in pan 15 minutes or just until firm.
Combine white chocolate and 2 tablespoons peanut butter in same bowl. Microwave at HIGH 1 minute. Stir well, and microwave 30 seconds, if necessary. Stir until smooth. Spread white chocolate mixture over first layer in pan, spreading until smooth with a narrow metal spatula. (Do not clean bowl.) Chill pan 15 minutes.
Place milk chocolate in same bowl. Microwave at HIGH 1 to 1 1/2 minutes; stir well. Spread milk chocolate over white chocolate layer. Shake pan to spread evenly. Sprinkle with peanuts. Cover and chill candy 1 1/2 hours or until set.
Score candy into 2" squares. Remove from pan by lifting candy and aluminum foil by foil handles. Cut candy into 2" squares with a long, thin-bladed knife. Place squares on cutting board, and cut each into 4 pieces.