Preheat oven to 375 degrees F (190 degrees C). Grease 12 muffin cups.
Bring water and salt to a boil; add oats and cook until thickened, 5 to 10 minutes.
Blend lemonade, cranberries, blueberries, and 3 tablespoons sugar in a blender until smooth.
Pour 1 cup of the lemonade mixture into a large bowl; add 1 cup sugar and the cooked oats and stir until just-combined. Discard remaining lemonade mixture or save for another batch.
Mix flour, baking powder, and baking soda in a small bowl; add to the lemonade mixture and stir until batter is smooth. Pour batter into prepared muffin cups to about 2/3-full.
Bake in preheated oven until lightly browned, about 20 minutes. Cool muffins in tins for 5 minutes before moving to a wire rack to cool completely.
