Hi All,
Thanks for the support,
We have found the solution, The LTO 3 tapes were not labled with Barcode that is the reason we could not configure the backup.
Created Barcode for the LTO 3 tapes and the tapes were detected while doing a Inventory and displaying in Media. Now we are able to take backup in both the tapes LTO 3 & 4 with out changing the hcart.