Preheat the oven to 375°F. Spray a 9x13-inch baking pan with nonstick spray. Set aside.
In a large bowl, combine the berries, sugar, cornstarch, and lemon juice. Stir together, and pour into the prepared baking pan. Set aside.
2 cups fresh blueberries, 2 cups fresh raspberries, 2 cups fresh blackberries, ½ cup granulated sugar, 1 tablespoon cornstarch, 1 teaspoon lemon juice
In a medium bowl, use a spatula, wooden spoon, or fork to combine the flour, oats, sugars, cinnamon, and butter. Stir together, until it becomes a crumble-like dough, and distribute the butter throughout the mixture.
½ cup all-purpose flour, 1 cup old fashioned oats, ½ cup brown sugar, 1 tablespoon granulated sugar, ½ teaspoon ground cinnamon
Using a spoon, drop small pieces of the topping over the berry filling, covering as much of the berry filling as possible.
½ cup unsalted butter
Bake for 28-30 minutes, until the topping begins to turn golden.
Allow the crisp to cool before serving as berries can be VERY hot.