Can I fast only on only coconut water / ash gourd juice / green juice the whole day?

Yes, absolutely. You can fast on only coconut water OR ash gourd juice OR green juice for 3 days. They are all very nutritious and cleansing for the body. 

However, if you are diabetic, strictly avoid fasting on only coconut water, as coconut water is too sweet for diabetes. Instead, stick to juices of non-sweet vegetables like cucumber, spinach, bottle gourd, ash gourd, etc.