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 1 or 2 warm matzo balls. For the matzo balls: 4 extra-large eggs, separated 1/2 cup good chicken stock, plus more for cooking the matzo balls 1/4 cup rendered chicken fat, melted 1/2 cup minced fresh parsley 2 teaspoons kosher salt, plus more for egg whites 1 cup matzo meal