What is the Overwrite Protection Period for the media set(s) involved
If you have three sets of tapes, and will reuse them every third week I would suggest a retention period of 19 or 20 days, not three weeks
If you really want Monday to go to Slot 1, Tuesday to slot 2 etc, you can set up partitions, and create or modify jour jobs to point to the partition rather than the loader as a whole See
http://seer.support.veritas.com/docs/206843.htm
http://seer.support.veritas.com/docs/262055.htm
http://seer.support.veritas.com/docs/241773.htm