How would you know the differences if misfiring because of a coil pack or valve

if it's a 4 cylinder,clear the code,swap coils with another cylinder,start the engine,CEL should come back on,see what the code is now. if it moved to a different cylinder,then you will know if it's the coil,COP. 6 cylinder only has one ignition coil per car. if it is a 6 cylinder,pull the spark plug wire from the plug on #3,set the plug wire close to a good solid place on the engine,start the engine,see any good blue spark coming from the plug wire?

I'm sorry but there is no way two different 'mechanics' could possibly be that mistaken about the culprit of a simple misfire .. That's just how it is! http://repairpal.com/repair-shops

As far as the ignition system goes, you can try to swap coils, and or spark plugs to see if the problem moves to another cylinder. As far as the valves goes, you can check compression, and perform a leak-down test on cylinder number 3.