As alvine said, this will not work and moreover it makes no sense. raid is not a backup! It helps nothing, if you delete a file by mistake and will mirror any virus directly to the other disk. My recommendation: use the 3TB disk as your main file storage, because it will be the fastest. Then use something like the SyncToy (http://www.microsoft.com/en-us/download/details.aspx?id=15155)
to copy the data to the other disks (1TB+2TB) frequently.