Place chicken in a 3-1/2- or 4-quart slow cooker. For sauce, in a small bowl stir together barbecue sauce, preserves, and mustard. Pour over chicken.
Cover and cook on low-heat setting for 6 to 8 hours or on high-heat setting for 3 to 4 hours.
Transfer chicken to a serving dish; cover and keep warm. Transfer sauce to a medium saucepan. Bring to boiling; reduce heat. Simmer, uncovered, about 10 minutes or until sauce is desired consistency. Serve chicken with sauce. If desired, garnish with fresh peaches.