3 pounds red potatoes, peeled and cut into 1-inch pieces
1/3 cup coarsely chopped fresh cilantro
1/3 cup coarsely chopped fresh dill
1/3 cup coarsely chopped fresh Italian parsley leaves
3/4 cup well-shaken buttermilk
1/2 cup plain Greek yogurt
1/3 cup mayonnaise
1/3 cup finely chopped scallions (green parts only)
3 cloves garlic, finely chopped
1 tablespoon dried chives
1 teaspoon kosher salt, plus more as needed
Place 1 1/2 to 2 inches of water in the bottom of a large pot. Place a steamer basket in the pot and add the cut potatoes to the basket. Bring the water to a boil over high heat and then reduce the heat to medium-high. Cover and steam the potatoes until knife-tender, stirring once or twice to ensure the potatoes in the center are evenly cooked, 25 to 30 minutes.
Transfer the potatoes to a large bowl. Let cool at least 30 minutes before dressing; the potatoes should be room temperature. Meanwhile, make the dressing.
Place the cilantro, dill, and parsley in a medium bowl and stir to combine. Remove 3 tablespoons of the mixture to a small bowl and set aside for garnish.
Add the buttermilk, yogurt, mayonnaise, scallions, garlic, chives, and 1 teaspoon salt to the medium bowl of herbs. Season with pepper and whisk to combine. Refrigerate the dressing until the potatoes are ready.
Pour the buttermilk dressing onto the potatoes and fold it in gently until the potatoes are fully dressed. Taste and season with salt and pepper as needed. Transfer to a large serving bowl. Scatter the reserved herb mixture on top.