Combine the rice, ham, turkey, pineapple, green pepper and water chestnuts in lightly buttered 2qt casserole. Melt butter in a skillet and stire in the soy sauce, garlic powder and pepper. Blend the corn starch with a small amount of the reserved pineapple juice. Stir this into the butter-soy mixture, cooking over low heat and stirring until mixture thickens slightly. Pour this over ingredients in casserole and toss to blend the sauce well. Cver and bake at 350deg about 30 minutes.