In a 2 quart baking dish, stir together plums, 1 cup sugar, water, tapioca and cinnamon. Bake in preheated oven for 25 minutes. (You can also cook the plum mixture in the microwave for 15 minutes.)
Raise oven temperature to 450 degrees F (230 degrees C).
In a medium bowl, stir together baking mix, 3 tablespoons sugar, milk and melted margarine to form a biscuit dough. Drop dough by spoonfuls onto plum mixture. Bake in preheated oven 10 minutes, until golden brown. Let cool slightly before serving.