A 6 with poison is really 3 hits, so 'that' hit is 3 hits with rending, so they all have to be saved at AP-4.
Deadly is its partner. A wound from deadly is really 3 wounds and would have to all get assigned to a single model (even if it's a waste/overkill) and would each have to get regenerated (3 rolls) in order to avoid some/all of them.
Edit: Blast is the same way. A hit with blast is really 3 hits and has to be saved 3 times.