For the moment the problem still seems to be on 3 cylinders and not a general fault - which means 'general' parts like the coil, air flow meter, should be ok...
You need to confirm if the injectors are firing on the cylinders where the problem occurs, cheapo stethoscope ( is that the right spelling ? ) from ebay is needed to listen to them clicking.
Injectors could be gummed up and need removing and testing on bench ( stick a load of redex into the tank )
Remove spark plugs and with good battery start car as normal ( ie crank it ), do you get petrol mist blown out of each spark plug hole ( small piece of tube to sniff through if not obvious )
Check the wiring to series resistors and the resistors themselves ( check connections )
Is there voltage at all injectors when running on 3 cylinders ?
You need to get the car running on easy start ( 2 man job, 1 man to control flow of easy start and other to keep car running - ) does its run more evenly ( ie all cylinders seem to be firing ). You may need to add easy-start to breather / or other port in intake manifold as airflow meter flap may block flow...
Could be a fault with the dizzy, as parts of it a cylinder specific..but they normally fail in a random fashion rather than knocking the same 3 cylinders out.
Have you done a compression test on all cylinders ?, could be valve / piston problem.
The injection system on the 280zx is simple so i would eliminate the obvious rather than paying someone at a garage to plug a 2015 code reader in to a 1970's ecu that doesn't have a code reader socket.
cheers