Make sure that your timing is on (pull covers, check marks) No, the t-belt is not part of a tune up, it's an expensive (or time consuming) job.
Do plugs, and while your at it, make sure there is spark at each before you start throwing more parts at the problem (coil packs) A simple inline spark tester will do (or a screw driver !) If there is no spark swap CP's around to be certain that you have a bad one. Unlikely to have three bad !
Next check that there is a pulse at each injector (stethoscope will work) If you have spark, and injectors are firing, next step IMO would be a compression test. It's not uncommon for high mileage EJ20's to exhibit misfires because of valve lash (out of spec) Low compression will confirm this. I kind of doubt that is going to be your issue, since you're showing misfires on more than two cylinders. I wouldn't be getting "on it" until the problem is solved, but if you have to depend on it for transportation, just baby it.