Do a lot of homework in here and Northstar Performance and read up on how to change the plugs - use only ACDelco #41-987 Platinum.
www.rockauto.com or Luke in Parts at Lindsay Cadillac over there >>>>>>>>>>> It's not the cat, and you're overdue for plugs by at least 10,000 miles.
Check the wiring connector at the ICM on each coil pack - the black area at the end of the pictured (front) coil cassette. RockAuto may have a few DELPHI coil cassettes with ICM left, but they're $185 apiece - one gray, one black.
Also check the fuel injector connector for each injector. A carefully loosened injector electrical connector is good for lots of diagnosis time........If your mechanic checked and cleaned
one plug he did one hell of a lot of disassembly for nothing, because #3 is on the rear side of the engine and requires a LOT of work to get to.