Deletes all data from the source that is older that (730) days
You can set the Backup Method in job properties to Full-Backup and delete the files or Backup Exec Archiving option. But i am doubtful about the second part of your requirement. I couldn't remember any option to set delete files older than x days.
So that on the media set I would always have the past (3) three years of data
How you are calcuating this? With one year retention, you can have 1 year data only.
and every day the > (3) year data on the media could be over written
If you are thinking about somethiing like overwriting some data on a particular media, sorry that is not possible. If i misunderstood your statement, explain it a bit better if you don't mind.
EDIT: With Archiving option, you can configure BE to archive files older than X days. As it is limited to disk only, you can configure a backup job to copy data and other valut store components ans databases to tape. You can also configure vault stores to delete files from original location after the vault store is backedup.