Preheat the oven to 400 degrees. Coat a muffin pan with cooking spray.
Prepare the muffin batter by whisking together the white sugar, brown sugar, and eggs until well creamy and smooth. Add the buttermilk, oil, and vanilla extract, then whisk until smooth.
In a separate dish, stir the flour, baking powder, and salt until well combined.
Slowly add half of the flour mixture to the batter and mix. Add the remaining flour and mix until just combined.
Gently fold in most of the raspberries, blueberries, and strawberries.
Pour the batter evenly into the muffin pan, filling them to the top.
Add the remaining few raspberries, blueberries, and strawberries to the top of each muffin, as needed.
Sprinkle the tops with some coarse turbinado sugar.
Bake for 5 minutes. Reduce the heat to 350 degrees and continue to cook for 13-15 minutes, or until a tester inserted in the center comes out clean.
Remove from the oven and cool in the pan for a few minutes before moving them to a wire cooling rack to finish cooling. Serve and enjoy.
Side Note: Baking at a higher temperature, then reducing it, helps the muffins dome.