Preheat oven to 350°F/175°C. Grease or use liners in the muffin pans.
In a large colander, combine the shredded zucchini and salt; mix well. Let stand for 10 minutes. Press as much liquid out of the zucchini as you can. Then transfer the zucchini to a clean kitchen towel and wring out any remaining liquid from the zucchini.
Meanwhile, melt the butter in a large bowl. Whisk in the honey and maple syrup until combined. Add the eggs and vanilla; mix well. Add the shredded carrot, apple, and zucchini to the bowl and stir until well combined.
In a small bowl, whisk together the flours, baking soda, baking powder, cinnamon, and nutmeg.
Add half of the dry ingredients to the zucchini mixture and stir to barely combine. Add the remaining dry ingredients and stir to barely combine. Do not over mix.
Divide batter evenly into the muffin pans; fill each to about 2/3 or 3/4 full. Bake for about 20 minutes, or until a toothpick inserted in the center of a muffin in the center of the pan comes out clean.
Notes
To make 1 cup oat flour, blend about 1 cup + 3 tablespoons of old fashioned oats in a blender or food processor until it reaches the consistency of flour.