Season the scallops and shrimp well with salt and pepper. Heat a 14-inch skillet over high heat. When hot, add 2 tablespoons of the olive oil. Add the scallops and shrimp and brown well on one side, about 2 minutes, then turn and cook about 1 minute on the second side. Transfer the shellfish to a platter and return the skillet to moderate heat. Add the remaining 1 tablespoon oil and the garlic to the skillet over moderately high heat. Sauté until the garlic begins to color, then add the Pernod. Simmer until the Pernod evaporates, then add the wine and simmer until reduced by half. Add the clams and mussels, cover, and raise the heat to high. Cook until they open, 3 to 5 minutes, checking occasionally and removing them to the platter as they open. Discard any that fail to open. Add the tomato to the skillet along with any accumulated shellfish juices on the platter. Simmer for about 2 minutes to soften the tomato, then reduce the heat to moderately low and stir in the tarragon and butter. When the butter melts, return all the shellfish to the sauce and reheat briefly. Serve immediately. | 
