I don't believe the DNS-345 explicitly supports RAID resizing. However, there are alternatives.
As per the DNS-345 manual, the DNS-345 supports RAID migration: Standard to RAID 1 without data loss. If you format the 3TB HDD in a free slot and copy the data from your RAID-1 array to the new 3TB standard volume, you should then be able to convert the 3TB standard volume to a new RAID array using one of the two physical HDDs from your old RAID-1. Not having worked with the DNS-345 I do not know the procedure for performing this function, but presume this resides under Disk Management.
Not knowing how smart the DNS-345 is, here is a trick used on the legacy DNS-343 series to prevent the unit from getting confused with the existing RAID-1 format on the old HDD your going to use for the RAID-1 migration: Mount the old RAID-1 HDD in a Windows PC and remove the existing partitions. Since the DNS-345 uses the Linux file system, having Windows alter the HDD will make the HDD unrecognizable to the DNS-345, prompting a reformat.
Presuming you go forward with this, please post back and let us know how things work out and describe the procedure for other readers.
** RAID-1 provides redundancy, but is not a backup (
DNS-345 - Data Backup Versus Redundancy). Make sure you have a second physical copy of your data before performing any procedure like this. If you don't have a backup, you can always use the 2TB HDD from your legacy RAID-1 array after copying your data from the RAID-1 array to the 3TB HDD,