The system was shut down, the new WD Green 3TB drive (about $130 from was installed and the system restarted.

Linux fdisk does not partition greater than 2TB so a new method had to be used to get this disk up and running.

try this instead of fdisk
parted /dev/sdb
mklabel gpt
mkpart 1 -1
mkfs.ext4 /dev/sdb1
done! A partitioned and formatted 2.7TB partition.

This was mounted and the process to copy all of the data from the two 1TB drives was started using rsync -auvxz /source /destination
It looks like it was moving about 40GB/hour and it took around 30 hours to completed the 1.2TB of data transfer.
Since new data was being added from various systems over this 30 hour time period, the rsync process was run again, this time in less than 5 minutes, to bring the 3TB drive up to date.

The drives were dismounted, rearranged in /etc/fstab and remounted again allowing the 3TB drive to be the primary data drive and the two 1TB drives to be partners in data backup.