I need some advice on a fuel injector problem

I had been out a couple of times one day and the last time i went to run an errand my engine light came on. I read the code which was P0203. I have replaced the fuel injector, the pig tail, new spark plugs, checked the coils and I get the same code after clearing the code. Which either leads to the wiring or the computer. both of which mean big bucks on my very limited income. A mechanic suggested that I find which injectors fired at the same time and piggy back the non firing injector to the good injector. So my question is... which injector fires with the #3 injector? Any help you can give me is deeply appreciated. Thanks