I had an injector missing on my GTP just last week. Go to a shop or if you know a mechanic that has a computer that reads codes, like a Tech2?? I think that's what they are called, or something like that, but you can hook it up to the wiring in your car and get a reading, also, take off the supercharger cover and unplug the wires that lead to the 3 missing injectors, the white, plastic plugins could be oozing fuel back up into the outlet, which is a sign that it's the injectors. My car was running sporadically too, it was an easy fix though.
