1998 Exhaust and O2 Sensor question.

I just bought a 1998 Dodge Ram 1500 5.9L Tuesday. After about 50 miles of driving it, the check engine light came on. Took it to Auto Zone and they showed 1 code and they claim that code showed the oxygen sensor after the Cat. They showed the truck took 3 sensors, 2 before the cat (one on each side) and one after the cat. Took the truck to a local exhaust shop that I trust and they said the Cat was all broke up, so I had them replace it. They cleared the check engine light and told me if it comes back on to bring it back in. After only 20 or so miles of driving it came back on, so I took it back, they said the oxygen sensors were bad and said it only has 2, one before & one after the cat and that both were bad, so I had them replace it. Check engine light is off again and hasn't came back on yet. I asked them what would cause the cat and both sensors to go bad and they said cats go bad every so often and that the cat going bad in return caused both O2 sensors to go back. My question is, is that true? How would the cat being broke up cause the sensor before the cat to go wrong? Is there a problem with the motor that would cause all 3 to go bad, as I've been told by a friend that its likely leaking fuel from the injector causing it to burn raw fuel and causing the cat to go bad and alll?

If you have a leaky injector, you'll likely get a miss which will turn on the Check Engine light.
Whatever happened, you have the bases covered regarding the exhaust and will likely have some trouble-free miles....I hope you have a warranty for the parts and labor.

If you have a leaky injector, you'll likely get a miss which will turn on the Check Engine light.
Whatever happened, you have the bases covered regarding the exhaust and will likely have some trouble-free miles....I hope you have a warranty for the parts and labor.

Thanks. So far the truck been running good and no more check engine light coming on. Yes there is a 2 year warranty on the cat & O2 sensors that cover both parts and labor. I did have a problem with the truck burning and leaking oil (leaking from valve covers dropping on to the exhaust manifold and burning off as well as the pcv valve was completely clogged causing it burn oil......got all of that fixed today.