We don't have the context on whether this has been working for you and
then stopped working, or whether you are trying something new, or
whether this is a problem with other types of operations on tape in
that library, or may be isolated to specific cartridges in that library.

I'd start by checking the device class assigned to your TSM db
backups, and assure that the drive type is consistent with the "L3"
tape at issue.  You can take a look at Technote 1303094 to see if that
pertains.  We don't know your library type, but possibly it is
configured such that it's operating in a 6-char volser mode rather
than 8, so the L3 is not being conveyed to TSM for media type to be
known.

   Richard Sims