Take a mixing bowl and pour in buttermilk, mayonnaise, and sour cream. Whisk everything together until the mixture becomes smooth and creamy.
Add Dijon mustard, lemon juice, onion powder, salt, and black pepper into the bowl. Stir well so all the ingredients blend evenly.
Put in the fresh chives, parsley, and dill. Gently mix so the herbs are evenly spread throughout the dressing without overmixing.
Taste the dressing and adjust the flavour if needed. Add a little more salt, lemon juice, or herbs according to your preference.
Pour the dressing into a clean jar or airtight container, cover it properly, and keep it in the refrigerator for at least 30 minutes before serving so the flavours develop nicely.