To Make Filling: In a medium bowl, combine blackberries, raspberries, and rhubarb. In a separate bowl, mix together 3/4 cup sugar and 1/4 cup flour. Sprinkle over fruit mixture and stir gently. Cover bowl and refrigerate overnight.
To Make Crust: In a large bowl, mix 2 cups flour with salt. Cut in shortening and 2 tablespoons butter until texture is like coarse cornmeal. Place 1/3 of mixture in a separate bowl. To the smaller portion, add water and mix to form a paste. Add this mixture back to the rest of flour mixture and stir just until dough forms a ball. Allow to rest at least 20 minutes before rolling out. Divide dough in half. Roll out bottom crust and place in 9 inch pie pan.
Preheat oven to 400 degrees F (200 degrees C).
Mix 1 tablespoon melted butter and lemon juice into fruit filling, then spoon into pastry-lined pie pan. Roll out top crust and place over filling. Crimp edges and cut steam vents in top. Brush lightly with half and half and sprinkle lightly with sugar.
Bake in preheated oven for 10 minutes. Reduce temperature to 350 degrees F (175 degrees C) and bake an additional 40 to 50 minutes, until crust is golden brown.