Why would you duplicate the data in the RAID array to a separate disk? That is what RAID is for.
Solution: Setup a 4 disk RAID 1 or RAID 10 solution, and install the OS on the array but on a separate partition. Also make sure you use a LVM for features like instant snapshots of your disk.
RAID 1 is a straight mirroring of your drives and duplicates all information across the array. You need a minimum of two disks and you must add disks in groups of two. Total capacity 146 GB. In this configuration you can handle from one to two disks failing without data loss.
RAID 10 combines mirroring with striping and gives you the same space as RAID 1 with the benefit of increased read and write speeds. However you need a minimum of 4 disks to set it up. Furthermore, you must add disks in groups of two. Total capacity 146 GB. In this configuration you can handle from one to two disks failing without data loss.
RAID 5 is slow for writes, and most of the time not worth the trouble in server environments, but if you want to squeeze some more space out of your drives, then you can setup a 4 disk RAID 5 solution. Here 3 disks would be used for data, so you would have a total capacity of 219 GB. You can handle only 1 disk failing.