Catalytic converter?

The misfiring dumps raw fuel into the exhaust, which is what can destroy them because they actually get too hot, and it damages the ceramic inside them. As such the misfires need to be repaired first. Then you would replace the converters. The only way a converter causes misfires is if it is plugged enough to restrict the exhaust flow. That would also cause other problems. If it was showing a P0300 but no individual cylinder codes, then I would look at the fuel system first. Low fuel pressure would cause a lean condition that would show as misfires.

Thank you for a prompt answer. Would changing the spark plugs/tuneup help with the misfires?

It might, but it depends on what the real cause is. You say it has multiple misfires, correct? That means it randomly misfires in different cylinders at different times. That is why I suggest the fuel system as the first start. It can alter all of the cylinders. The ignition system, however, has 6 plugs, 6 coils, and 6 injectors. To get a P0300 means that all 6 of a given item started to fail at the same time. What are the chances of all of one item just suddenly failing all at the same time? However low (or high) fuel pressure, or an intake air leak (brake booster, broken vacuum line or intake gaskets all would affect all 6 cylinders.