Usually with tape devices you don't have to do anything special to overwrite them - simply start writing again from the beginning of the tape, and whatever was already on there is gone.
As a FAQ for one classic backup program notes, the problem is usually the reverse: most tape devices are so keen to rewind to BOT and start overwriting their payload, that considerable design and implementation effort is expended on preventing them from doing exactly that:
When appending to a tape, there is the possibility that, between the
time that Amanda positions to the last image (that already is not
really trivial!), and opening the device for writing, a tape rewind
happens, and in that case Amanda would happily erase ALL of the tape,
containing possibly many days worth of backup.
(AMANDA solved that problem by simply never appending to a tape - you write everything you're going to write to that tape in a single pass during which time the device is kept locked. Draconian and wasteful, yes, but that gives you some idea how easy it is to accidentally rewind and overwrite a tape!)
I would advise against degaussing for anything except secure destruction (and I wouldn't rely on it for that, either). As Wikipedia notes:
For certain forms of computer data storage, however, such as modern
hard drives and some tape backup drives, degaussing renders the
magnetic media completely unusable and damages the storage system.
This is due to the devices having an infinitely variable read/write
head positioning mechanism which relies on special servo control data
(e.g. Gray Code) that is meant to be permanently embedded into the
magnetic media.
[...]
The servo patterns are normally never
overwritten by the device for any reason and are used to precisely
position the read/write heads over data tracks on the media, to
compensate for sudden jarring device movements, thermal expansion, or
changes in orientation. Degaussing indiscriminately removes not only
the stored data but also the servo control data, and without the servo
data the device is no longer able to determine where data is to be
read or written on the magnetic medium.
though I freely admit I don't know whether DDS-3 is one such medium.