How to Make It
Make the crumble topping: Mix oats, flour, brown sugar, pecans, and salt in a medium bowl. Add butter and knead until mixture resembles coarse crumbs.
Make the cake: Preheat oven to 350°F. Lightly butter a 13-by-9-inch baking dish. Dust with flour and tap out excess. Using an electric mixer, beat butter on medium until creamy, about 2 minutes. Add granulated sugar and beat until light and fluffy, about 3 minutes. Add eggs, 1 at a time, beating well after each addition.
Whisk baking powder, salt, and flour in a bowl. With mixer on low, add flour mixture to butter mixture in 3 additions, alternating with milk, beginning and ending with flour mixture, beating well after each addition. Beat in vanilla. Transfer batter to prepared baking dish.
Mix blueberries, raspberries, rhubarb, and 2 cups crumble topping in a large bowl; sprinkle mixture over batter. Top with remaining crumble topping.
Bake until a toothpick inserted in the center of cake comes out clean, 65 to 70 minutes, Let cool and dust with powdered sugar before serving.