Combine Ragi Flour and Rice Rawa. Pour about 3 cups of water and let it soak for 4 hours.
Wash Urad Dal and soak it in 2 cups of water for 4 hours.
After 4 hours, grind the Urad dal to a fine paste. Add more water if needed.
Add this dal paste to the soaked Ragi and Rice Rawa.
Mix well. Ferment it overnight in a warm place.
The next day, add salt. Mix well. Add water if necessary. The batter should be neither too thick nor too thin.
Heat water in an Idli steamer. Meanwhile, grease the idli plates with oil or ghee. Put portions of the batter in each section of the idli plates.
When the water in the steamer starts bubbling, put the idli plates in the steamer.
Steam on high heat for 15 minutes.
After 15 minutes, turn off the heat and let the idli plates stay in the steamer for about 5 minutes. Then remove the plates and keep them on the kitchen counter for about 2 minutes.
Sprinkle some water on the plates. That will ease removing the idlis.
Remove the idlis and serve them hot with chutney and/or sambar.