RAID5 can work with 3 disks. What was described however was the use of 3 disks, 1 of which was a dedicated parity disk. In other words, 2 disks are data and 1 is partity for the 2 disks. That is a RAID3 configuration. Note that it's not that there are 3 disks that makes it RAID3, it's the configuration (dedicated parity disks) that makes it so. You can use RAID3 for any number of disks above 2 in fact. You could have 5 disks that use RAID3 - 4 data disks and 1 parity disk. Anything above 5 requires more than one parity disk however (in principle you get nothing for using 6 disks). RAID3 does have it's uses but I personally think (and I'm sure do many other people) that it's not the best use of your group of disks because of the potential performance problems. It's not so pronounced with a 3 disk config but if you took a 5 disk config, it would mean that any data write would always require the dedicated parity disk to be writen to as well. In other words, the parity disk is 4 times busier than any other disk in the array. It's probably for this reason that RAID5 was introduced. RAID5 has virtually all the same constraints as RAID3 except that the parity data is deliberately distributed to all disks in the array. The available storage from the array is the same and the raitio of parity to data is the same but the hot parity disk is eliminated. RAID5 is still not optimal for performance but it's a good deal better than RAID3.