Do you have an installation manual for the tachometer? From your description it sounds like there are three separate holes and each hole has a potentiometer in it. Is that correct, or is it one adjuster that has three positions? If this is a universal tachometer C4, C6, and C8 should relate to selecting the number or cylinders of the engine, but I'm not sure why they would appear to be adjustment potentiometers for each one unless there is another switch or jumper to select the cylinder count.
-Rod