Houghten nailed the confusion that is here.
First you roll armor pen, then reroll any fails.
Rerolls mean that the first roll never happened.
Then, you know how many 6es are there, and you add a d3 to them, specifically.
The USR says that every 6 allows "a further d3 to be rolled", further as in relation to the d6 that is an armor pen roll. So now the armor pen roll consists of a d6 and a d3, and Pask allows you to reroll the d3s as well.
If the d3 was a Rending roll and not an armor pen, as previously argued, it would not have any effect. As nowhere does the rules cover that rending rolls have any effect on armor, only armor pen rolls do
Maybe re-reading the paragraph on rerolls will also clarify this.