You ONLY get an earth when it fires, there will no doubt be a bit of stray earth in there, but not enough to do anything other than read on a digi meter, as soon you try to draw on it it will drop to zero.
If at no time you are getting a 12V switched earth, on each injector in turn, then the ecu is not firing the injectors.
Just disregard the 3v reading, it's of no significance, unless it were switching at 3v but then that would be only with the engine turning over.
tabetha