I wanted to let you know that i tried 3 TB hard drives in my NAS (N5200B Pro with Linux Ubuntu server 10.04.2) and it did not succeed. Why? well one thing is that i doesn't matter what kind of OS you install its hardware related.
Because of this problem Thecus NAS will probably not support 3 TB hard drives when it comes to hardware. On the software side the Linux kernel has to be "2.6.1" or higher. I'm not sure what kernel the Thecus Linux has, but this problem can easily be solved by installing your own Linux flavor for example Cent-OS, Ubuntu, fedora, etc.
I switched the 3 TB hard drives with 2 TB hard drives of Western Digital.
edit: the type of 3 TB hard drives that i used were the ones of Western Digital ( WD30EZRX )