Preheat the oven to 350. Combine the oats and brown sugar in a small bowl. Spread the mixture in a baking pan and bake until it turns light brown, 8 to 10 minutes. Cut the margarine into small pieces and scatter them in the pan. Return the pan to the oven until the margarine has melted, about 1 minute. Stir the oats to coat them with the margarine and bake the mixture for 5 minutes more. Set the oatmeal topping aside to cool. (The topping may be made ahead and stored, tightly covered, for several days.)
Put 1 cup each of the blueberries, raspberries, and blackberries in a 2-quart bowl and set aside. Combine the lemon juice with the sugar in a saucepan and bring the mixture to a boil. Add the remaining cup of blueberries to the syrup, reduce the heat to low, and cook for 3 minutes. Add the remaining cup of raspberries and cup of blackberries. Bring the mixture to a simmer and cook, stirring constantly, for 3 minutes.
Pour the cooked fruit into a sieve set over the bowl of reserved berries and use the back of a wooden spoon to press the fruit through the sieve. Stir gently to coat the whole berries with the sauce. Spoon the warm fruit mixture into individual ramekins or small bowls. Sprinkle some of the topping over each portion.