Rinse and drain salmon, shrimp, and scallops. Cut salmon into 1-inch chunks.
In a food processor, whirl seafood to a thick paste. Add egg whites, salt, cayenne, and nutmeg; whirl until blended. Then add cream and whirl until very smooth.
Butter 8 molds or ramekins (1/3- to 1/2-cup size). Spoon seafood mixture equally into molds and spread level.
Pour 1/2 inch of boiling water into a 12- by 17-inch baking pan. Set molds in water and cover pan tightly with foil.
Bake in a 350° regular or convection oven until a metal skewer inserted in center of timbales comes out clean, about 15 minutes. With a wide spatula, lift timbales from water. If done up to 30 minutes ahead, cover loosely with foil and let stand at room temperature.
Spoon Tarragon Beurre Blanc equally onto eight warm plates (see notes). Run a knife between timbales and sides of molds. Invert a mold over sauce on each plate to release timbale; remove mold. Garnish timbales with fresh chives.
Tarragon Beurre Blanc: In a 2- to 3-quart pan over high heat, boil 1/3 cup white wine vinegar, 1/3 cup dry vermouth, 3 tablespoons minced shallots, and 1 1/2 tablespoons chopped fresh tarragon until reduced to 1/4 cup, 5 to 8 minutes. Let stand at room temperature up to 2 hours.Just before serving, set pan over high heat, add 3/4 cup (3/8 lb.) butter, and stir constantly until it's melted and bubbling, 2 to 3 minutes. Serve warm.