1. Preheat oven to 350°.
2. Weigh or lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and the next 4 ingredients (through salt) in a large bowl, stirring with a whisk. Add grated apple, grated carrot, shredded zucchini, and walnuts to flour mixture; toss well. Combine canola oil, buttermilk, and eggs in a small bowl, stirring with a whisk. Add egg mixture to flour mixture, stirring just until combined. Spoon batter into an 8 x 4–inch loaf pan coated with cooking spray. Bake at 350° for 50 minutes or until a wooden pick inserted in the center comes out clean. Cool 10 minutes in pan on a wire rack; remove cake from pan. Cool completely on wire rack before slicing.