Preheat the oven to 350°F. Spray a 10-cup bundt pan or 12-cup bundt pan with baking nonstick spray. Set aside.
In a large mixing bowl, sift the flour, baking powder, baking soda, and salt. Set aside. Zest the three lemons into a small bowl. Juice one of the lemons in a separate bowl. Set aside.
3 ½ cups All-purpose flour, 2 teaspoon Baking powder, 1 teaspoon Baking soda, 1 teaspoon Salt, 3 Large lemons, ¼ cup Freshly squeezed lemon juice
Using a mixer, beat the butter, oil, and sugar. Beat on high speed for 3 minutes. It will be very light and fluffy. Add lemon zest, lemon juice, and sour cream. Mix on medium until combined.
1 cup Unsalted butter, 2 tablespoon Oil, 1 ½ cups White granulated sugar, ¼ cup Sour cream
Add eggs in one at a time. Mix on medium until combined. Make sure to scrape the bowl with a rubber spatula.
4 Large eggs
Add in ⅓rd of the dry ingredients and half of the buttermilk. Mix on low speed. Add another ⅓rd of the dry ingredients and the rest of the buttermilk. Then, finish with the rest of the dry ingredients.
1 cup Buttermilk
Fill the bundt pan ¾ths full. If using a 10-cup bundt pan you will use only ¾ths of the batter. Don't overfill the pan.
Bake for 40-50 minutes until a toothpick inserted comes out clean.
Let the cake sit in the hot pan for 10 minutes. Then, flip the cake onto a cooling rack. Cool the cake completely.