Preheat oven to 350 degrees F (175 degrees C). Grease a 9 inch cake pan. Combine the flour, baking powder and salt. Set aside.
In the prepared pan, combine 3 tablespoons margarine and brown sugar. Place pan inside the preheated oven until the margarine melts and begins to bubble. Set aside. In a large bowl, cream together the 1/3 cup margarine and 1 cup white sugar until light and fluffy. Beat in the egg, then stir in the vanilla. Beat in the flour mixture alternately with the milk, mixing just until incorporated.
Arrange plums around the edges of the prepared pan, overlapping slightly. Distribute the blueberries in the center. Pour batter into prepared pan, covering fruit completely. Bake in the preheated oven for 40 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool 15 minutes before serving.