Missing and backfiring at 3k does not seem like it would be a dirty injector or a dirty air filter. You said the plugs are new, did the problem start when the new plugs were installed, or maybe shortly after? Plugs could be the problem. Maybe spark plug wires are crossed? Maybe a bad spark plug wire? It could be a lot of things. I would pull the plugs, check the gap on all, and make sure none of them look bad, and make sure none of them look like that cylinder was running lean. Then when you put them back in, make sure all the wires are in the correct place, and that all of the wires are in good shape. Doing this is free so it can't hurt. Do this, if it doesn't fix the problem, then look elsewhere. That is what I would do.

Make sure your alternator and battery are in good shape, too. You may be suffering from an undervoltage, and have low spark, too.
You may also want to check for codes using the procedure up in Techinfo.

well i changed the plugs and wires the day after i bought it in May 03, the batter was in there when i bought it, im going to try taking the new battery out of my park avenue just to see if it makes a diffrence, im going to now changed the fuel filter and PCV and run some STP through it. And eventually look at my plugs and wires.....this is the nice thing about having a non used park ave with a 3.8 in it sitting around...the park ave also has new plugs and wires, im gunna transfer all the wires over and try that as well. then remove the plugs gap em, and look for any carbon tracking or anything out of the ordonary.

i get pretty bad milage on my ssei but i think its becaus eim "testing it" since i got my new intake i tend to stomp on it...so i dont get goo dmilage...i get like 230 when im stomping on itt...about...i use stp fuel cleaner for 21gal tanks and stp octane booster everytime i get gas!!!!