Add the soaked sabudana along with the water to the mixie. Grind it in a mixie to a smooth paste. Set aside to a bowl.
Add the soaked and drained urad dal and add upto a cup of ice water to grind. Grind to a smooth fine paste. Set aside to a bowl.
Grind the drained rice in 3-4 batches. Add upto 1 cup of water while grinding. Remove the ground rice mixture and add it to the vessel.
Add in the salt and mix the batter well with hands.
Allow the batter to ferment for 8-10 hours. It might take longer if you live in a cold place. Mix the idli batter after it has fermented and store it in the refrigerator and use it within the next 3 days.
Take about 3 cups of batter in a bowl. Add in a pinch of baking soda and mix well.. Baking soda is an optional ingredient.
Put some water in an idly vessel and put it on medium flame. Oil the idli plate and gently fill the rounds with the batter. Put it inside the idly vessel and let it steam for 6-7 minutes on medium flame.