Soak kokum in 3 tbsp warm water. The water will turn a reddish pink. In a blender, grind 3 kokum, garlic, green chilli and salt, gradually adding some water (about a tablespoon at a time). Add coconut milk. Dilute with more water as per taste. Place it in the refrigerator until well chilled. Pour out into serving glasses, sprinkle chopped coriander leaves on top and serve.