I have logged fault codes P0300, P0301, and P0303. What is the problem?

Thanks to all of you for your help! I ended up getting help from a friend who is a mechanic. He first drained the fuel tank and put in some shell premium. Following that the only code that remained was P0303. So he swapped injectors 1 and 3 and the code moved to 1. He installed a new injector in 1 and everything appeared fixed. Except a day later I was driving along and got 2 cat codes, P0420 and another. I don't happen to recall exactly right now. But I was afraid of something like this because I drove about 100 miles with those misfires!! So now I have some more research to do. I am really hoping this doesn't lead to a new cat before christmas!! Not at all what I asked Santa for.

Have the 02 sensors tested with a scan tool, mechanic can tell from live data if cat is failing. Possible downstream 02 fault.

2.4L 4 cylinder! Has no "other bank".

Bank '2' is where all the guesswork$$$ goes to!

Very well put!