Place the chicken in a large resealable plastic bag set in a shallow dish. In a small bowl whisk together the lemon juice, oil, and the salt; pour over chicken in bag. Seal bag; turn bag several times to coat chicken. Chill for 30 minutes. If using wooden skewers, soak in water while chicken marinates.
Add tomatoes, onion wedges, and bread cubes to bag and turn bag to coat ingredients with the marinade.
To make the kabobs, thread 2 bread cubes and 2 tomatoes alternately onto each of 12 skewers, leaving 1/4-inch gap between pieces. Repeat with the onion and chicken, threading 2 onion wedges and 2 pieces of chicken onto each of the remaining 12 skewers. Grill on the greased rack of a charcoal or gas grill directly over heat. Grill bread and tomatoes for 1 to 2 minutes or until browned, turning once. Grill onion and chicken for 10 to 12 minutes, turning occasionally, or until onion is tender and chicken is no longer pink. Serve each person 2 to 3 of each kind of kabob.
To serve, use a fork to push food off of the skewers onto plates.