1. Preheat oven to 375°F. Line muffin tins with paper liners.
2. Sift flour, baking powder, baking soda, salt and cinnamon together in a large bowl. Stir with your hand (or a whisk) to be sure the ingredients are combined. In another bowl, combine the milk, eggs and melted butter.
3. Make a well in the middle of the dry mixture, pour the wet mixture into the well, and stir until just combined. There will be some lumps but don't over-mix the batter.
4. Add the blueberries, raspberries, strawberries and sugar and stir gently to combine.
5. Using a large spoon or scoop, spoon batter into the muffin cups to fill the liners.
6. Bake for 20 to 25 minutes, until a cake tester comes out clean and the tops are nicely browned.