If I understand correctly you have a 3TB drive with only the 2TB allocated in the partition, and you want to re-size the disk to 3TB?
You can do this with a partition manager/editor software in a few minutes.
I personally use "gparted", it is a linux partition editor that is free and works very well. You can find it here http://gparted.org/
It is a Live CD with a Graphical User Interface so it is very easy to use, even if you are new at doing this sort of thing.