Preheat the oven to 350ºF and grease a 9x5-inch loaf pan.
In a large bowl, whisk together the 3 large ripe bananas, 3 large eggs, and ¼ cup oil, then stir in the 2 cups whole grain flour, ¾ cup granulated sugar, 2 teaspoons baking powder, 1 teaspoon ground cinnamon, and ½ teaspoon fine sea salt, stirring until a smooth batter forms. Fold in the 1 ½ cups fresh raspberries. Pour the batter into the prepared pan, smooth the top, then rest the batter until the oven is fully heated.
Bake the loaf for about 60-70 minutes, checking at 60 minutes, when a tester comes out clean or with a few moist crumbs.
Cool the bread in the pan for 20 minutes, then cool it on a rack. I like to cool the bread completely so that the crumb sets, but it's OK to cut into it after about 30 minutes (it's delicious warm!).
Notes
Can I use frozen bananas? Absolutely. A great way to save ripe bananas is to freeze them in a freezer-safe bag. When you're ready to bake, take out as many bananas as you need, thaw in a bowl, and drain any liquid before mashing them up.
Reduce the sugar. If your bananas are very, very ripe and sweet, you can reduce the sugar by ¼ cup (so, ½ cup of sugar). You can give the batter a very small taste test to determine when you do need the full ¾ cup of sugar.
Rest the batter. Don't turn the oven on until you have assembled the batter. While the oven is preheating, the flour will have time to hydrate, making for a softer, better crumb after the loaf is baked. This is true whether you are using gluten-free oat flour or whole wheat flour; whole wheat flour also benefits from a rest before baking.
Can I use all-purpose flour? Yes, of course! Use 2 cups of all-purpose flour for the whole grain flour.
Try other spices. Instead of cinnamon, use ground ginger or cardamom.
Plus vanilla. Stir in 2 teaspoons of pure vanilla extract or vanilla bean paste when putting together the batter.
Make it refined sugar-free. Substitute coconut sugar or maple syrup for the granulated sugar.