after fitting new spark plugs. on cylinder 1, 2 and 4 are darker on cylinder 3 it is almost unchanged. practically clean. if there is no spark, it should be flooded and it looks as if it has no fuel or too little fuel. swapped injections and the same. problem with the injection cable? give the ecu for checking? or how to check if fuel is dosed there?
ECU is starting to sound more and more likely to be the culprit. I don't know if there's a way to test, but perhaps check if #3 injector is pulsing correctly? If it is, there may just be a blockage, though I know you've replaced most of the fuel components. Maybe a local dyno can tell you if the ECU is OK? I'm still invested in this problem as I don't know the the resolution, so keep us posted.