In a large bowl whisk together the 1/2 cup warm water, the molasses, oil, and yeast. Cover and let stand about 5 minutes or until the mixture is cloudy and a little foamy.
Add the 1/3 cup water. Add 1 1/2 cups of the all-purpose flour, the rye flour, buckwheat flour, and salt. Stir until nearly combined. Turn out onto a lightly floured surface. Knead in enough of the remaining all-purpose flour to make a moderately soft dough that is smooth and elastic (5 to 7 minutes total).
Punch down dough. Turn out onto a lightly floured surface. Grease a 8x4x2-inch loaf pan. Shape dough by gently patting and pinching into a loaf shape, tucking edges underneath. Using a paring knife, make a few diagonal slashes in the top of the loaf. Place shaped dough in the prepared pan. Cover; let rise in a warm place until nearly double in size (about 30 minutes).
Preheat oven to 350 degrees F. Brush dough with melted butter. Bake in the preheated oven for 40 to 45 minutes or until mahogany in color and bread sounds hollow when lightly tapped. Remove from pan and cool on a wire rack.