What kind of RAID did you want to create? Creating a RAID0 across all three drives will hurt rather than help your data security and while a RAID1 across two drives makes some sense I think RAID1 across three drives is a little silly. I usually RAID1 two of the drives and install the system on that partition and leave the third drive for data storage. That gives you separation of data and system but gives you a second system drive to boot from if the other system drive kicks out.
To create a RAID5 across the three internal drives you'll need hardware like an internal RAID card as well as the drives and if your budget is tight it might be a stretch, but that's up to you. If you run RAID5 without a hot spare you're fooling yourself if you don't have a "cold spare" on the shelf at all times. This will also only give you 2 drives worth of storage space (RAID5 is n-1 storage) which makes it more expensive per byte.
As always RAID is not back-up - that is a separate issue. Different RAID schemes protect your data in different ways. Mirroring doesn't protect you from file deletion, failed updates etc. and RAID5 will protect you against the lose of a _single drive_ but not more. So it depends on what you want and you're willing to/can spend.
What you say makes sense. I guess I was wondering how people use three drive spaces effectively, since as you point out, you can't do a software RAID 5, "0" doesn't help security, and "1" just needs two drives. On my older G4 with four drives I created a RAID 0+1, which can be done with Tiger. So I'm leaning towards striping two (for speed), and then doing backup to a the third. I know back-up should really be seperate, but I think this is a practicle level I can live with.
If you don't have data on the System drive that changes a lot - like a mailserver, databases or web activity - then the system is much easier to replace. Be sure you export your OD and your server settings regularly and you should be good to go.
In my situation the "real data" lives on fibre DAS RAID so my internal third "data" disk really doesn't have anything crucial on it - the one or two most recent files I can about I could get from back-up if I wanted them.
Everyone's set-up is different - except everyone needs back-up,