In a mixing bowl combine buttermilk, mayo, sour cream, chives, parsley, dill, lemon juice, celery salt, garlic powder, onion powder, salt and pepper. Reserve a ⅓ cup for dipping later.
Place the chicken on a baking sheet and pat it dry. Take the ranch dressing and rub it all over the chicken. Cover the chicken with plastic wrap and refrigerate for at least 4 hours or up to overnight for best results.
Next, preheat the oven to 425 degrees. Spray a 10-inch skillet with cooking spray. Add in the potatoes, carrots, and onions. Season with salt, pepper and olive oil. Place the chicken on top of the vegetables and roast for 15 minutes in the oven. After 15 minutes, place a piece of foil on top of the chicken. Continue cooking until an internal thermometer reaches 165 degrees, which is about 1 ½ hours.
Once cooked, remove chicken from the oven. Let cool before slicing and serving with ranch dressing.