How in the world did you fill 3GB so quickly (is someone downloading craploads of pr0n :-)? Something is funky about what's being backed up...
You can attach a new external tape drive (depending on what interfaces that machine has) and then set it up to back up to the new tape drive instead. The company should be able to do that remotely if you plug it in. Examples (eSATA or SCSI external ports on your server):
http://www.newegg.com/Product/Produc...82E16840108047
http://www.newegg.com/Product/Produc...82E16840108016
If the company does get in remotely again, you should probably ask them where in the world all your backup space is being used up. They should be able to do that basic "directory of backed up data" thing I asked you about before...post it here so we can see what's going on...