I heard about Andhra bank Arogyadaam which even covers pre existing diseases after completion of 3 years of start of the policy. Can any ne guide me in this?

I'm not sure where you're located but yes, most health policies have a "blackout" period where pre-existing conditions will be covered after the policy has been in force for 3 years.

If you are looking for a plan to cover certain condtions (ie. Cancer policy), then be prepared to pay a much higher premium.