Preheat oven to 350ºF. Lightly grease a 9 x 13'' casserole dish. Arrange scallops, shrimp, and crabmeat in a layer along the bottom of the dish. Heat three tablespoons of the butter in a medium skillet. Add mushrooms and sauté for three minutes. Spoon into casserole dish and top with a layer of cheese. In a small saucepan, melt four tablespoons butter over low heat. Add flour and salt and simmer for one minute. Gradually add milk and stir until thickened. Pour into casserole dish and sprinkle bread crumbs over top. Bake 25 minutes. Note: Can be assembled before baking one day in advance.