What was his reason for prefering the mirror? He may think write performance will be lower with RAID 5 and there's a parity calculation overhead, but I don't think you'd find the performance a problem and your CPU will be quick enough for the parity calcs. RAID 1 (mirror) is simplier, but if you've got three identical disks you may as well use them in one large array unless you want two 2TB drives in Windows (1 2x 2TB RAID 1 and the individual 2TB disk).
Rich is indeed correct, but I find myself sat in three camps on this.
Software RAID has it's problems in life and will always be a compromise to full hardware controller based RAID. I've used software based RAID 5 setups in the past and got my fingers burnt, due to some computer hardware problems.
I'd almost be tempted to go with the mirror option. It improves read speed to a degree, but means you arn't putting all your eggs into one basket. I'd work it like this:
SSD - Boot drive / applications / user profile / sppeeeeeeeeeeeeeed
HDD 1 + 2 - Mirror RAID configuration (a good place to store your stuff, improved read speed and has redundency). Gives 2TB of space to play with.
HDD 3 - Used as a backup to the backup possibly? Or could be used for non-essential stuff.
It's always handy to have that drive that isn't part of the RAID set. What happens in the future if you reaaaaaaaaaly need to get some data off that machine in a nice easy way? The RAID array doesn't transport so easily, but the sing drive would nicely.
It's up to you.....I can see pros/cons for each way myself. How much storage space do you need and how important are your backups and data?
(sorry...this may not help)
BG
Arguing with a woman is like reading a Software Licence Agreement.
In the end, you ignore everything and click "I agree".
Either RAID 1, or RAID 5 provide you with protection in the event of a disk failure. The idea is that if a disk fails everything keeps working as normal. You just power down the system and replace the failed disk. The data is then copied from the remaining disks in the array onto the new disk to provide you with full protection again.
Not sure what he means by major problems if one disk goes down as that's the whole point of RAID!
You still need backups to provide you with protection from viruses etc. RAID just provides disk failure protection. If you're still unsure I can explain further.
As Billy says, you could put the 2x 2TB in RAID 1 and remove the third 2TB disk from your system and put it in an external enclosure e.g. eSATA for backups.
In my personal system I run 1x SSD for apps etc., 2x disks in RAID 1 for data, music photos etc. and 1 disk in an external enclosure for backups.