Since you haven't done so, I'd replace the fuel filter as well...just so you can rule it out. But, it sounds to me that you are having a spark problem in the #3 cylinder. If it was getting spark, when you pulled the plug wire from #3 cylinder, the engine would not sound the same. However, the fact that you have replaced the cap/rotor/coil and still have the same problem in the same cylinder is very interesting. And since you swapped the injector as well and still have the same problem rules that out.

As for the fuel pressure idea, if that was the problem, it would be the same in all cylinders...so it's definitely a spark problem. If you wanted to test fuel pressure, you'd need a gauge. I know B&M sells a filter top gauge that comes with the correct banjo bolt for it.

I'd try swapping out the distributor if you know someone with the same OBD version GSR as you. At least you would be able to rule out the distributor even if it was still giving you the same problem.

I guess it could be an ECU problem as well...so if swapping distributors doesn't help, try running another GSR ECU. Maybe something funky is going on there...shit happens sometimes.

Good luck, and keep us updated.