The simple answer should be to use your backup to restore the data to the new 3TB RAID 1, but by the sounds of your request you don't have a backup - not a good idea as RAID 1 is NOT a backup strategy - read the sticky 'data backup versus redundancy' above.
There are two ways you can go.
Easiest is to make a backup of your data then remove the two 2TB drives - mark them left and right just in case - and replace them with the two 3TB drives. Then use the web interface to setup the new drives as RAID 1. After they are formatted and visible on the network restore your data from the backup.
The harder way, since you use a mac and not windows, is to read the sticky 'data recovery (windows PCs)' and see if you can adapt the information to your equipment or, maybe, you could borrow a windows or linux PC to do the job.
If you decide to do this and need more information just ask.
I do recommend that you use a USB/SATA caddy or adapter to plug in ONE of your RAID 1 drives to the PC, leave the other drive in place until you are sure you can read the data on the drive.