In a large bowl, gently toss together blackberries, raspberries, blueberries, and white sugar; set aside.
In a separate large bowl, combine flour, oats, brown sugar, cinnamon, and nutmeg. Cut in butter until crumbly. Press half of mixture in the bottom of a 9x13 inch pan. Cover with berries. Sprinkle remaining crumble mixture over the berries.
Bake in the preheated oven for 30 to 40 minutes, or until fruit is bubbly and topping is golden brown.