Whisk together mayonnaise and sour cream in small bowl. Slowly whisk in buttermilk, paying attention to thickness of dressing. If you like your ranch on the thicker side you may want to use slightly less than 1/2 cup of buttermilk.
Whisk in lemon juice, pepper, salt, and garlic powder.
Whisk in Italian parsley, chives, and dill.
Cover with plastic wrap and refrigerate until well chilled, about 2 hours.