To Make Stuffing: Melt 2 teaspoons butter or margarine in a skillet. Add the bread crumbs. Saute and stir the mixture over medium-high heat till the bread crumbs are browned. Remove the bread crumbs to a mixing bowl.
Melt 1 teaspoon butter or margarine in the skillet and saute the onions, celery and garlic until tender; add to bread crumbs in mixing bowl, then stir in shrimp, crab, parsley, salt and pepper and toss gently.
Cut foil to form a double-thickness 18x12-inch rectangle. Lay the fish fillets on the double thickness of foil. Mound the stuffing on top of the fillets.
Curl up the edges of the foil to form a tray. In a covered grill, arrange the preheated coals at either side of the grill. Test for medium heat above the center of the grill. Place the fish in foil in the center of the grill rack. Cover and grill for 20 to 25 minutes or just until the fish flakes easily.