It would help to know how many DRIVES are involved.
Here's one option:
1. COPY the data to another drive (external or USB)
2. Remove the 1TB RAID
3. Create the 3TB RAID
4. COPY back the data
This is probably the easiest solution. If you have enough drives to REMOVE one from the RAID5 setup, you can use it to copy the data to.
Western Digital also has a network solution which is Gigabit capable. The "LIVE DUO" 6GB is RAID1 at 3GB effective or no RAID for 6GB. That probably doesn't interest you but it's worth mentioning. It idles at 5W only and communicates via Ethernet (up to 128MB/second if router is Gigabit capable).
The advantage of the LIVE DUO is that it can be SHARED with any other device on your home network such as a compatible HDTV media player (DLNA) to stream movies or music. You can also setup an automated backup solution using Acronis True Image from any PC.