In a medium bowl, whisk together mayonnaise, sour cream, and buttermilk until smooth. Stir in parsley, dill, vinegar, Worcestershire sauce, garlic powder, onion powder, salt, and pepper. Blend well and adjust seasonings, to taste. Cover and refrigerate for at least 4 hours or overnight. If consistency is too thick, thin out dressing with a splash of additional buttermilk.