Cook, bake, or microwave the three kinds of potatoes until done but still nice and firm. Plunge into cold water. Let stand until cool enough to handle, then peel and cut into ½- to ¾-inch dice.
Combine the potatoes in a mixing bowl with the celery, bell pepper, olives, olive oil, vinegar, dill, salt, and pepper. Stir together gently.
Stir half of the arugula in with the potatoes and line a serving platter with the rest. Mound the potato salad onto the platter, and scatter the seeds over the top. Serve at once.
