A P0303 is a misfire in the #3 clyinder, the Kia Tech site list the possible causes as:

Faulty spark plug or wire
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector
Burned exhaust valve
Faulty catalytic converter(s)
Running out of fuel
Poor compression
Defective computer

A compression test will tell you if there is a pressure leak in the clyinder, but not if it's from a valve not seating correctly, a burnt valve or piston ring issues. You can get a compression tester for around $30

I had my entire top end rebuilt, all 4 intake, and 4 discharge valves replaced and re-seated, new guides, skimmed the head, new head gasket, water pump and timing chain etc! for less than $2,000 at my Dealer and thats Canadian $$ so that would be around $1500 to $1800 USD.

You can make your own spark tester, to see if a spark is getting to the plug, and for the actual cost of a new plug I wouldn't run the old ones. Sprk Tester

A tell tale sign that a valve is not seating correctly is a ticking sound from the upper engine, excessive oil and or gas comsumption, black soot in the tail pipe and black or gray smoke in the exhaust. Not all these signs would be present at the same time unless both a intake and discharge valve were faulty!

If it were me, and it was a few years back, I'd make a spark tester to see if the coil pack was firing, if it was then I'd change the plugs (and the seal) If it wasn't I'd change the pack then change the plugs and see what happens!