Soak both the rice and urad dal separately overnight.
Drain and grind them separately using water as required to a smooth and thick batter.
Mix the three batters well in a deep vessel. Place the yeast in a bowl with sugar and two tablespoons of warm water. When it begins to froth add it to the batter and whisk well.
Add coconut milk and salt and mix. Set it aside to ferment for about five hours or till it doubles in quantity.
Heat sufficient water in a steamer. Grease the idli moulds. Pour the batter into each mould and place it in the steamer.
Cover the steamer with the lid and steam on medium heat for about fifteen minutes or till done.