Preheat oven to 375°.
In large bowl, combine flour, baking powder, sugar, and salt. Cut or grate butter into flour mixture. Then, stir in sage, apple and gruyere until well distributed. Set aside.
In smaller bowl, combine two eggs and buttermilk. Whisk until well combined. Make a well in center of flour mixture, pour in egg mixture and stir until it comes together. Whisk additional egg in small bowl combined with one Tbsp buttermilk. Set aside.
Turn out on lightly floured counter and knead a few times until it holds it's shape, adding additional flour if needed. Divide into two equal portions and roll into an 8 or 9 inch circle. Dust bottom of the circles with flour.
Cut each circle into 6 to 8 triangles. Place triangles on parchment lined baking sheet. Lightly brush with egg wash, then bake for 15-25 minutes at 375°, or until golden and cooked through.
Cool on a wire rack until cool enough to handle. Serve.