Preheat your oven to 350ºF and grease a 9x13-inch baking dish.
In large bowl, stir together the strawberries, blackberries, and blueberries. Add the starch and lemon juice, and stir to combine. Refrigerate the mixture for 20 minutes.
In medium bowl, whisk together the flour, baking powder, and salt.
In a mixing bowl, beet the brown sugar, buttery sticks, and vanilla extract on medium speed with a mixer until combined. Gradually add the flour mixture and blend, until crumbles form.
Pour the berries into your prepared baking dish. Evenly top the berries with the crumbles.
Bake the crumble for 30 to 40 minutes, or until lightly browned.
Serve topped with dairy-free ice cream, if desired.
Recipe by Go Dairy Free at https://www.godairyfree.org/recipes/dairy-free-mixed-berry-crumble