I’m Not A Pro Mechanic, So Take This Advice Accordingly. I Believe This Kia Has CVVT (Constant Variable Valve Timing).
Are you getting a fault code for a misfire on cylinder #3 and is that the only code(s) you have retrieved ?
Since switching coils around didn’t help, maybe the problem is with the valve timing system, even the OCV (Oil Control Valve) component.
Perhaps the reason the new plugs helped a little is because the mis-fire fouled a plug or plugs a little.
A Kia TSB (Technical Service Bulletin) written for Kia technicians says that even using an aftermarket oil filter can affect oil flow rates and they say to make sure the filter is OE not aftermarket. The flow rate may affect the OCV and therefore the CVVT.
The bulletin goes into a lot of detail on how to test an OCV before finding it faulty and a candidate for replacement. I would not assume this part is bad on this particular Kia. More diagnosis is needed first. Throwing parts at it is not a good plan.
You say the “light came on” , but do you know if any diagnostic codes were set that could help ? I’d have somebody retrieve any codes and write them down exactly as indicated. Some auto parts (chain stores) will do this for free in their parking lot.
Related Diagnostic Trouble Codes:
Codes P0011, P0012, P0014, P0015, P0021 and P0022 relate to camshaft position and timing.
Codes P0016, and P0018 relate to crankshaft position correlation.
Code P0300 relates to Random / Multiple Cylinder Misfire Detected.
Maybe this would help narrow the problem down a bit. You are concentrating on the igniton part of a misfire.
Get any code(s) and post them here.
I’m beyond my level of expertise here, so maybe a pro will weigh in and bail me out.
CSA