Preheat oven to 350°F (175°C). Line or grease a 12-count muffin tin.
In a medium bowl, whisk together all dry ingredients.
In a large bowl, whisk together all wet ingredients until smooth.
Fold grated zucchini, carrot, and apple into the wet mixture.
Add dry ingredients to wet and stir gently until just combined (don’t overmix).
Fold in nuts/raisins if using.
Divide batter evenly into muffin cups (about ¾ full).
Bake 20–23 minutes, or until a toothpick comes out clean.
Cool in pan for 5 minutes, then transfer to a rack.
For glaze: whisk ingredients and drizzle over cooled muffins.