We don't really know which layout the existing two drives use. You can show this with
zpool status
in the Shell.
Ignoring how this might be set up currently, expanding the capacity safely would probably involve making a full backup of the data, destroying the existing volume, installing the new drive, and then creating a RAIDZ1 of the three 1TB drives.
FreeNAS 9.3 is very old now. FreeNAS 11.1-U1 is the current version, and it would be good to consider upgrading if your hardware meets the minimum requirements.
FreeNAS 9.3 User Guide:
http://doc.freenas.org/9.3/freenas.html
FreeNAS 11.1 User Guide:
http://doc.freenas.org/11/freenas.html