It's a good time to change your spark plugs and maybe wires and do it today because a misfire will dump fuel into your exhaust and fry your catalytic converter making this a more expensive repair. Just stick with platinum ones if you plan to continue running a stock motor. P0303 is cylinder 3 misfire, it's the center cylinder towards the front of the car and at the very least change that plug now.
If new plugs and wires properly installed don't solve your problem then the misfire is caused by a bad fuel injector, I doubt a bad coil pack because cylinder 6 is not misfiring.
