Preheat oven to 350 degrees.
In a medium bowl using a rubber spatula, gently toss together the blackberries, raspberries, blueberries, and white sugar; set aside.
In a separate medium bowl, combine flour, oats, brown sugar, and cinnamon. Add the butter in pieces and carefully blend while keeping crumbly.
Coat 6 (3 ½-inch) ramekins with cooking spray, see prep tip. Divide the sugared berries among them.
Divide and sprinkle evenly over the berries the crumble mixture, about ⅓ cup for each.
Line a baking sheet with parchment paper or foil. Place the 6 ramekin cups on the pan.
Bake for about 40 minutes until golden brown. Top with a little whipped cream or frozen yogurt, if desired.