I'm thinking ECU. The ECU has two big output transistors that fire the injectors in banks of three, and if one of those transistors burns open, you will lose those three injectors.
There aren't a lot of other things that could cause you to drop three injectors like that at the same time. As you already noticed, the dropping resistor does four and two so that's probably not it. And while it is possible that the wiring to the front three of the injectors is screwed up inside the harness, I consider it unlikely. Why? Because each injector gets it's own independent wire from the ECU to the injector. So sure... It's possible that a PO cut three of those wires, but I consider it more likely that the ECU is the root cause.
I'm in the Philly area if you get stuck. I'm no FI expert, but I know some stuff. PM me if you need a hand.