Correct, 1st you need to create a new Aggregate with 3 drives and then copy root volume vol0 to it. ( As highlighted in the KB article above).
If by any chance you don't have 3 Disks you can do the same with RAID-4 and later on out of the reclaimed Disks can convert it back to RAID-DP.
Or if this option also not available you can simply destroy the Aggregate and build from it from CF card image.
-Bakshana