Place the chickens, onions, carrots, celery, parsnips, parsley, thyme, dill, garlic, and seasonings in a 16- to 20-quart stockpot. Add 7 quarts of water and bring to a boil. Simmer, uncovered, for 1 hour. Remove 2 of the chickens and allow to cool slightly. Remove the breast meat from both chickens and set aside. Return the remaining chicken and carcasses to the pot and continue simmering, uncovered, for 3 more hours. Strain the entire contents of the pot through a colander and chill. Remove the surface fat, then reheat the stock as follows, or pack in containers and freeze.

To serve the soup, return the stock to the pot and reheat, adding the diced carrots, celery, dill, and parsley. Shred the reserved chicken breast meat into large pieces and add to the stock. Simmer over low heat for 5 minutes to cook the vegetables and reheat the chicken. Season, to taste, and serve as is, or ladle each serving over 2 warm Matzo Balls.