Preheat oven to 350 degrees F (175 degrees C). Grease a 13×9-inch baking dish.
Mix 2 cups flour, 1/2 cup brown sugar, and 1/2 teaspoon cinnamon together in a bowl. Cut 1 cup unsalted butter into the flour mixture until crumbs form; press into bottom of prepared baking dish.
Bake crust in preheated oven until golden brown, about 15 minutes.
Blend eggs, white sugar, 1/2 cup flour, and salt in a blender until smooth; pour into a large bowl. Fold rhubarb, strawberries, and blueberries into the blended egg mixture. Spread the fruit mixture onto the baked crust.
Mix 1/2 cup flour, 1/2 cup brown sugar, oats, melted butter, and 1/2 teaspoon cinnamon together in a bowl; sprinkle over the fruit mixture.
Bake in preheated oven until lightly browned on top and the filling has set, 45 to 50 minutes.