Preheat oven to 350 degrees. Lay down a round piece of parchment for a 10 inch cake pan. Butter and flour the sides of the pan. Cut the 5 tbsp butter into small chunks and scatter them in the bottom of the pan. Set aside. Slice one of each orange and lemon into thin slices. Slice half of the pink grapefruit and set aside the other half. Set on a large plate and pour the 2/3 cup of sugar over them. Let sit for 5 minutes. Lay citrus slices in an even layer in the bottom of the pan and set aside. For the cake batter: Add olive oil and the sugar and mix until creamed together. Mix in one egg at a time. Add the citrus zest, and juices of the second orange and lemon and from the other half of the grapefruit. Add vanilla and mix until just combined. Add in the flour, baking powder and salt. Mix, then add buttermilk. Pour batter over the citrus slices in the cake pan. Make the crumb topping: In a small bowl, stir together the flour and sugar. Add the butter and mix with fingers until different size crumbs form. Sprinkle the crumbs over the cake batter. Bake for 50 minutes to 1 hour. Let the cake cool in the pan for 10 minutes, then run a knife around the edge to loosen and then invert the cake onto a cooling rack and cool.