I just accomplished something similar. If you want to follow along what I did 
here is the forum: 
http://www.backupcentral.com/phpBB2/two-way-mirrors-of-external-mailing-lists-3/symantec-netbackup-18/staging-off-virtual-tape-library-106811/

basically my solution was to run a script before the weekend backups would run. 
the script would deactivate the duplication portion during the weekend. and 
then Monday morning another script would re-activate the duplication portion 
and it would begin immediately then. Thus letting you schedule that third stage 
when you wanted.

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