While triple parity has been one of those topics that's been under the radar for several years, NetApp was awarded United States Patent 7,640,484 on December 29, 2009 ("Triple parity technique for enabling efficient recovery from triple failures in a storage array").
In fact, just this month (January 2010), ACM Queue published an article entitled, "Triple-Parity RAID and Beyond" by Adam Leventhal. Note Figure 1 in the document ("Comparison of RAID-5 vs. RAID-6 Reliability") is actually a NetApp-created chart from a few years back:
(SIDE NOTE: extra credit if you can calculate the probability of data loss with 2TB SATA disks under RAID-5! )
Back to the patent now: So NetApp techies are familiar with "diagonal parity" in RAID-DP (if you're not, read more here: http://media.netapp.com/documents/wp_3298.pdf). With this patent, NetApp takes it one step further with a novel approach to triple parity encoding using anti-diagonal parity sets -- quoting:
"…the anti-diagonals have a slope that is orthogonal to the diagonals, providing a uniform stripe depth and an amount of parity information equal to three disks worth" (minimum required to reconstruct from a triple disk failure).
Also of interest is the technique for triple parity reconstruction (Section F). Again, reference the above patent for details.