Information is helpful when requesting help...
SAE OBD-II DTC P0303 is a cylinder #3 misfire (just to let everyone else know. You can look them up here -
http://www.geocities.com/JeepI6Power/obd2.html - I've gathered them from various Jeep FSMs. I also have OBD-I DTCs listed - separate page.)
It would also help to know what year your rig happens to be - since 1996-1999 came with conventional electronic distributor ignition, and 2000-2001 uses "coil rail" ignition (no distributor.) This makes troubleshooting a bit difficult.
However, it isn't always a spark plug proper that can cause a misfire. I'd check for a good connection between the spark plug and spark plug terminal on the HT lead (you should hear an audible "click" when fully seated. Also, visually check the inside of the terminal for corrosion that may be interfering with connection.) This would apply to both conventional distributor and COP/coil rail ignition. And, if you still have a dizzy, pull the #3 wire and check resistance - I think the "nominal" spec is something like 1000 ohms per foot of wire, for "resistance" wires (like we've got.)
Have a "noid light?" They only cost a few bucks, and you can use them to check to make sure the fuel injector is getting a trigger pulse from the ECU. The injector coil proper may also be checked with an ohmmeter - I don't recall the spec, tho.
But, first and foremost, it really helps to give what information you have to hand - and, if you've been working on the problem, chances are you've already looked up the code. I was feeling rather charitable - else I'd have just said "give us more information" instead of looking it up for you. There's really no excuse not to - it's out there (you can find a lot of it on my site, and I'm planning on adding a good deal more as I accumulate it.) Also, giving with year/engine is
very important, since it tells us right away what generation of controls are used, and there's got to be
someone who knows what the quirks of your year happen to be. Even if you don't know what your DTC indicates offhand, there is
no excuse to not be able to give with the basic vehicle information - so you may consider yourself read off for that one (sorry, but that's just the way it are. Basic troubleshooting relies upon gathering information, so give with what you already know. Saves us the trouble of asking...)