hi Abu,
Can u send more specific info please, has Netbackup version, Oracle Version, RMAN log error and if is posible create/check the dbclient, bpcd and bpbkar logs under your client, also take a look at the media server logs on bptm, normaly error 3 or 2 are comunication issues with the database from Netbackup and this ca be due the script you are using to perform the backup or DNS problems.
Please let us know
regards