Preheat and prep pan: Preheat oven to 350°F. Line a 9x5-inch loaf pan with parchment paper.
Mash bananas: Mash 3 ripe bananas (about 1½ cups mashed) in a large bowl.
Add wet ingredients: Add 1 cups shredded zucchini (do not wring out the liquid), 2 large eggs, lightly beaten, 2 teaspoons vanilla extract, and 1 teaspoons lemon juice (or white vinegar); mix to combine.
Add dry ingredients: Add 2 cups rolled oats, processed into flour, 1 teaspoons baking soda, 1 teaspoons ground cinnamon, and 0.3 teaspoons salt; mix until just combined. Batter will be thick.
Fill pan: Transfer batter to the prepared loaf pan.
Bake: Bake for 55 minutes
, or until a toothpick comes out clean. Check at 35 minutes and loosely tent with foil if browning too fast.
Rest in pan: Let the loaf sit in the pan for 20 minutes
Cool and slice: Transfer to a wire rack and cool for another 20 minutes
before slicing.