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Originally Posted by silicon212
I think that you're barking up the wrong tree with the sensor replacements. If there's a cylinder misfire, it's not going to be caused by the oxygen sensor. If it's a common #3 misfire, I'd look at the plug, coil, injector or even things such as valve leaks or a blown head gasket (not likely). This misfire will screw with the oxygen sensor, causing the PCM to set a DTC and lighting the MIL.
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Agreed, the misfire is setting the 420 code as the unburned oxygen in the exhaust stream fools the post cat sensor into thinking the cat efficiency is down, usually 420 is not set with a hard 0300 failure, but as silicone212 says, focus on the misfire issue.