Preheat oven to 350 degrees F (175 degrees C). Grease an 8x8-inch square baking dish.
Mix strawberries and blueberries with 1/4 cup sugar in a bowl, and set aside. Mix the cream cheese with the marshmallow creme in a separate bowl until smooth, and set aside. In another bowl, mix the baking mix, milk, 3 tablespoons of sugar, and melted butter into a smooth batter. Pour the batter into the prepared baking dish.
Bake in the preheated oven until lightly golden brown on top, 12 to 15 minutes. Allow to cool in the pan.
Spread the berry mixture and juice over the cooled shortcake, saving a few pretty berries aside for garnish. With a spatula, gently press down on the berries to allow the juices to soak into the cake. Spread the cream cheese mixture over the fruit, and garnish with the reserved berries. Chill in refrigerator for at least 30 minutes before cutting in squares to serve.