Blend the rice coarsely in a blender. Blend even the dals coarsely in a blender. Soak all the 3 powders in Luke warm water adding sufficient salt. Make sure to add the water just enough to cover the mixture (make the batter a little thicker than the dosa batter).Let the mixture set for about one and half an hour. Add cumin, green chillies, coconut, coriander & crushed black pepper to the mixture & mix well. Mean while heat a griddle & grease it with a tsp. of oil. Ladle a big spoon full of batter into the center of the pan and spread it around in a circular fashion (from inside out) shaping the batter into a thick dosa. Wait for few seconds and then take a tsp. of oil and spread it over the edges of the dosa. When the edges turn golden, flip the dosa on the other side. Again add a tsp. of oil & cook it for a minute. Serve the dosa with butter.