Melt 3 tablespoons butter in a heavy saucepan over low heat; add flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add milk; cook over medium heat, stirring constantly, until thickened. Remove from heat; set aside.
Melt remaining butter in a large skillet; sauté oysters, crab meat, shrimp, salt, pepper, and hot sauce 5 minutes or until edges of oysters curl. Stir in white sauce and sherry. Cook 1 to 2 minutes. Spoon mixture evenly into patty shells. Serve immediately.