Any further failure would result in a complete data loss. Also, if any of the RAIDZ stripes loses more than one vdev simultaneously, this will lead to a complete data loss as well.
Previously, I would have used straight ZFS mirrors for performance and expandability (raidz VDEVs can't be expanded), but at this scale, the 9-disk 3x3 raidz1 setup consistently outperformed a 10-disk RAID mirror setup in random and sequential I/O.
In this case, going to raidz2 across all 12 disks would give an extra disk worth of capacity/performance with the ability to lose any 2 disks without data loss and going to raidz3 should give the same capacity/performance with the ability to lose any 3 disks without data loss.