On your countertop, lay out a sheet of parchment paper and top it with a layer of plastic wrap.
Lay out your fillets of salmon, tails away from you. Overlap the fillets of flounder about 1 inch over the tails of the salmon.Place the scallops on a metal skewer and set at the end of the flounder furthest from you. Sprinkle the herbs over the fish and season with salt and pepper, to taste.
Using the plastic wrap, roll the fish towards you so that the plastic begins to pull the flounder over the scallops. Be sure not to roll the plastic into the fish roll. Use a sheet pan to push the roll tight as you pull the plastic toward you. The roll should be tight and you should be able to remove the sheet of plastic. Rolling away from you, wrap the fish in the parchment so it is covered and can be placed into the refrigerator. Refrigerate for 1 hour.
Make the compound butter: In a large bowl, mix the butter, parsley flakes, salt, and pepper using a wooden spoon. Place the mixture on a piece of parchment and fold the parchment over itself. Pull to form a roll and twist the ends. Place in the freezer for 10 minutes to set up. Slice into 1/4-inch rounds and remove the parchment.
Heat your broiler and place the oven rack 6 inches from the heating element.
Remove the metal skewer and slice the roulade into 3/4- to 1-inch thick rounds. Place onto a broiler pan and brush each round with canola oil. Put under the broiler for 3 to 6 minutes, depending on how well done you like your fish.
Serve with 1 slice of compound butter on each fish roll.