Preheat oven to 375 degrees F (190 degrees C).

In a large bowl, toss the sliced rhubarb with granulated sugar and 3 tablespoons of all-purpose flour until evenly coated.

Spread the rhubarb mixture into an ungreased 9x9 inch baking dish.

In a separate bowl, combine the rolled oats, brown sugar, 3/4 cup all-purpose flour, and ground cinnamon.

Cut the cold, cubed butter into the oat mixture using a pastry blender or two knives until the mixture resembles coarse crumbs.

Sprinkle the crumble topping evenly over the rhubarb in the baking dish.

Bake in the preheated oven for 40 to 45 minutes, or until the topping is golden brown and the rhubarb filling is bubbling.

Remove from the oven and let cool for at least 10 minutes before serving.