In a 2 quart baking dish, stir together plums, 1 cup sugar, water, tapioca and cinnamon. Bake in preheated oven for 25 minutes. (Or 15 minutes in microwave.)
Preheat oven 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 shortcake dough. Drop dough by spoonfuls onto baked plum mixture. Bake in preheated oven 10 minutes, until golden brown. Let cool slightly before serving.