Preheat the oven to 375 degrees F (190 degrees C). Lightly grease a 9x13-inch baking dish.

Place rhubarb in a large mixing bowl. Add white sugar and 3 tablespoons flour; stir well to coat and spread evenly in the bottom of the prepared baking dish. Set aside.

Combine 1 1/2 cups flour, brown sugar, and oats in a separate mixing bowl; cut in butter until the mixture is crumbly. Sprinkle crumble mixture over rhubarb layer.

Bake in the preheated oven until rhubarb filling is bubbling and crumble on top is golden brown, about 40 minutes.