Wash the chana dal, urad dal, and toor dal together and soak it for 1 to 2 hours. Soak the red chilies along with the dal. On festive days, I first soak the lentils and then start preparing the other dishes. I make the vadai as the last thing. You can soak the lentils for about 2 to 2.5 hours.
Wash the moong dal separately and soak it separately for the same time.
Grind the lentils and prepare the batter-
After 2 hours, drain the water and add the chana dal, urad dal and toor dal along with the red chili. Add the salt, thing, and turmeric powder. Grind it into a semi-smooth consistency. If needed, sprinkle some water and grind it. You might not need more than 2 tbsps of water for this batter.
Transfer this into a bowl. Now drain the water from the moong dal and add it to the batter. Add the curry leaves and also the cilantro. Mix it well.
Fry the vadai-
Heat the oil for frying in a kadai. Drop a small piece of vadai batter into the oil. If the oil is hot, the batter will rise immediately and sizzle.
Wet your palms and shake off the excess water. Take a small gooseberry size vadai mixture and flatten it into small patties and slowly drop it in oil. Let it fry for 30 to 45 seconds and slowly flip them. Fry until the vada turns golden brown. By this time, the sizzling sound will reduce as well. Drain the fried vadai on a kitchen towel. Depending upon your pan size, you can fry 3 to 4 in one batch. Similarly, flatten and fry the vada with the remaining batter.
Adjust the salt and spices according to your preference.
Don't grind the batter too smooth nor too coarse. While grinding, carefully add water. Add the water only if required.
Adding soaked moong dal gives a nice texture, but that's optional.
If you are not doing it for festivals or as a part of neivediyam, you can add ¼ cup of finely chopped onion and some fennel seeds to the vada batter like masala vadai. Instead of onion, you can add finely chopped cabbage or carrots or spinach as well.
Always fry the vada in medium heat. Don't overcrowd your pan with too many vadas.