[my reply is about seeing only 2TB on your 3TB drive... as others replied, only 1 drive is recognized by the TabloTV DVR, even if you hook up a USB hub etc... update May 5, 2018]
You need to "repartition" your 3TB drive so that you can use the full 3TB on your WD Red drive. Once you partition your WD Red drive to have one big partition, the TabloTV "format" should provide 3TB of storage. The Tablo will re-format the 3TB partition with the filesystem it needs to have.
Your WD Red Drive may have a "Master Boot Record" [MBR] partition map. My TabloTV drives are set up with a "GUID" partition map [GPT].
If you want to try to copy your 1TB drive to your 3TB drive - here is what I followed to pull this off... https://community.tablotv.com/t/how-to-migrate-to-a-bigger-drive/1177/72
Basically you boot Linux from a DVD and then hook up your freshly formatted 3TB drive and your original Tablo Drive. This flavor of Linux knows the filesystem used by TabloTV, and then a file by file copy should get your info over, but a terabyte takes a long time to copy on a USB 2 interface.
Be careful, you do NOT want to repartition your main computer drive, just the USB attached drive.