In a large saucepan, combine the white wine, tarragon, and oregano; bring to a boil over high heat. Add the clams; cover and cook for 4 to 5 minutes, or until the clams open (start checking after 3 minutes). Remove the clams as they open (discard any that do not open); reserve the liquid in the pan. When the clams are cool enough to handle, remove the clam meat and place in a large bowl; discard the shells.
Heat the liquid in the pan over medium-low heat. Add the shrimp, and cook for 3 to 4 minutes, or until cooked through. With a slotted spoon, remove the shrimp, reserving the liquid in the pan. When the shrimp are cool enough to handle, halve them horizontally and add to the bowl with the clams.
Heat the liquid in the pan over medium-low heat. Add the scallops, and cook for 2 minutes, or until cooked through. With a slotted spoon, transfer the scallops to the bowl, reserving the liquid in the pan. Cool the shellfish cooking liquid to room temperature and measure out 1/2 cup.
In a medium bowl, whisk together the 1/2 cup reserved cooking liquid, the lemon juice, oil, and mustard. Pour the dressing over the shellfish. Add the celery, bell peppers, and parsley; toss to combine. Chill the salad until serving time.