06 Hummer H3- check engine light

Unfortunately that is old technology. You do have an option of possibly a parts store reading the trouble codes, at often times no cost. If it had misfires, I would probably guess that spark plugs was not the cause. The question is, is when it misfires? Early H3's(2006) had issues with cylinder heads causing valve leakage, and one way to detect this is monitoring the misfire counts with a scan tool. If the counts would continually increase as the engine idled, and would stop when the speed was raised, then it was almost always the case that a valve was leaking, and this was confirmed by a cylinder leakage test(not a compression test). This is not the only possibility, but a common one. There could be a faulty ignition coil, but then again, you would need to see the counts and determine which cylinder was at fault. From there you could swap the coil(and plug if needed) to a cylinder that wasn't clocking misfires, and see if the misfire now follows the coil. If it stays with the cylinder that it was originally on, then most likely it is internal. I believe there was a service bulletin on this condition.