2005 Monte Carlo P0300 Misfire After Replacing All Three Coil Packs - What's Next?

I took my 2005 Monte Carlo to a repair shop, where they diagnosed multiple misfires using OBD-II code P0300. They concluded the issue was caused by two faulty coil packs and recommended replacing all three for full coverage. I ordered two coil packs online but later decided to replace all three at a local auto parts store. The technician advised I bring in the parts so they could perform the replacement themselves, especially since I had a holiday weekend planned when their shop would be busy. After installing all three new coil packs, the same P0300 code and misfire symptoms returned. I then visited a local parts store that offers OBD-II scanning, but their scanner is a basic model and only reads general codes — not detailed fault data. As a result, I'm back to square one. I previously had both the oxygen sensor and mass air sensor/collector replaced in January due to a check engine light issue, which was required for vehicle inspection. If the mechanic confirmed faulty coil packs and all three have been replaced, what could be causing the persistent P0300 code? Any other potential causes or diagnostic steps? Thanks in advance for any insight! Brian