It sounds as if you may have a Duo (v1) which will not support drives larger than 2TB. It would give you 2TB if they were supported but not fully usable. Unfortunately the 3TB drives require a formatting type (GPID) that the Duo cannot do. It only works with MBR drives.
You have several options on how to proceed. However, to find out which one is available, a few more facts are needed.
1. How long have you had the Duo?
2. If you have had it a while and had other drives in the unit, how did you remove them? a) With the unit turned off or b) with the unit on, replacing them one at a time with the 3TB drives. (I am concerned about retrieving any data that may have been on the unit).
Remember - the NAS is not a backup plan.
Backup = Good, No Backup = Bad