For stuffing, melt the 2 tablespoons butter or margarine in a skillet. Add bread crumbs. Cook and stir over medium-high heat until bread crumbs are browned. Remove bread crumbs to a large bowl.
Melt the 1 tablespoon butter or margarine in the skillet. Add green onions, celery, and garlic. Cook over medium-high heat until tender. Add mixture to bread crumbs in bowl.
Add shrimp, crabmeat or scallops, parsley, salt, and pepper to bowl. Toss gently.
Cut foil to form a double-thickness 18x12-inch rectangle. Lay fish fillets on the double thickness of foil. Mound stuffing on top of the fillets.
Fold up edges of foil to form a tray. In a covered grill, arrange preheated coals at either side of the grill. Test for medium heat above the center of the grill.* Place the fish in foil in center of grill rack. Lower grill hood and grill for 20 to 25 minutes or just until the fish flakes easily. Makes 4 to 6 servings.
To test for medium heat, you should be able to hold your hand at the height of the food for 4 seconds before you have to pull away.