Set the three drives with three BI folder. Set each folder to delete. Based upon the size of the drive and the size of the camera resolution, evenly distribute the cameras to the drives so that they all will be deleted at about the same time. DO NOT MOVE FILES BETWEEN DRIVES OR FOLDERS.
I can't seem to find where I can split cameras up to record to a particular drive. It just appears a global setting in "Clips and Archiving" tab... that's whats tripping me up right now.