DO-AHEAD TIP: If using bamboo skewers, presoak them in water for 30 minutes, to prevent burning on the grill. Preheat grill to HIGH.
In a large bowl, toss together all ingredients. Alternately thread chicken, apple and onion onto skewers.
Reduce grill heat to MEDIUM-HIGH; place skewers on grill and cook for 10 minutes per side or until chicken juices run clear and apple slices and onion wedges are tender.