2001 Pontiac Grand Am

Hello, I'm tryng to help my friend fix her car. It's a 2001 Pontiac Gran Am GT 3.4L with 113,000 miles. I did a complete tune up replacing spark plugs/wires and even the fuel injector for cyl #3. It still bogs/stalls/hesitates when you accelerate and Check Engine Light code still reveals P0303. At one time it did generate a P0300 which I understand it means multiple missfire. I was thinking it could either be the ignition coils or module, but before making any unneccesary expenses for her. What do you suggest?
March 15, 2007.

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