It may be good to post the figures up on the HC and CO. It may be close to what Daniel's 80 was doing, were it passed HC and CO but failed NOX because the O2 sensors were not working well and leaning out the fuel mixture too much. So if engine is not getting enough fuel, it can create what you are looking at. If you have a infared temp sensor and you can test the temp before and after the cats to see if you are getting any temp difference that would be good. But if not, then the assumption one can make is the the cats may not be getting enough fuel to lite off to burn at the proper temps to clean up the exhaust going out the tail pipe.
But if you state that one cylinder's injector is possiably (being open) over fueling #3 cylinder, then the O2 sensor on that bank can be over whelmed with fuel and be the computer could be leaning out the other cylinders to the point that it can create a lean problem and then have nox go up as well as not having enough fuel to allow the cats to work properly. How many cats and how many 02 sensors does it have. 1 cat and one sensor or 2 cats and 3 sensors or some other combination.
So I would make sure you do not have a bad injector on #3 as well as test the O2 sensor to make sure they are working well, before I replace the cats.
Maybe this helps