Ok,you'll need a multimeter that has an ohm or continuity tester on it.
Then,pull the plug off injector no3 and test it out to the multiplug at the end of the fuel rail,that way checking the integrity of the wiring to the injector on the ohm/continuity setting.
If that seems ok,put the tester on dc volts and put each probe into the connectors at the end of the offending plug and,with the car running see if there's any voltage getting there,you can even put the tester on ohms across the injector itself to see if you get a reading,there should be something not open circuit or dead short,best thing to do is remove the plug on the one next to it as well and see if the readings tie in,should be roughly the same.
Also,double check that the little spade connectors where the injector plugs in (female) aren't damaged in anyway as that will cause the problem as well.
It's pretty easy to test overall,just needs a bit of patience.
If you can,test the injector loom right back to the ecu to prove that wiring out as well,may need a wiring diagram for that or make a good note of the colours to each injector,the returns should all be the same or the feeds depending if it's earth or live switched,but do t worry too much about that for now.
If the wiring and injector all check out then it's most likely the ecu although in all my time with ka's I've NEVER heard of this problem,or anything ecu related tbh.
The tamperproof screw is to prevent theft as all you need then is the ecu and matching key to steal any ka.