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.
If you don't have any buttermilk on hand, you can make your own by pouring 1 1/2 teaspoons of lemon juice (or white vinegar) into a 1-cup liquid measuring cup. Fill the measuring cup up to the 1/2-cup line with milk (whole milk is preferred but low-fat/skim milk will work). Allow to sit for 5 to 10 minutes and stir before proceeding with recipe.
To make Ranch Dip (instead of dressing), use a total of 3/4 cup mayonnaise, 3/4 cup sour cream, and eliminate the buttermilk.