If you are looking at the X3 head on, move to the dovetail on the right side of the machine. On the very top of the head you should see a big fat flat headed screw (about 3/4" diameter). There should be a matching one on the bottom of the head.
Unscrew the bottom screw about 3 1/2 turns. WARNING: AT THIS POINT, DO NOT MOVE THE Z AXIS OF YOUR X3 OR YOUR GIBB MAY FALL OUT OR BECOME VERY LOOSE. THIS WOULD NOT BE A GOOD THING!!!
Now, screw the top screw in as far as you can (Finger Tight, NOT Gorilla tight). On the Gibb, there is a little step it the top of the flat screw rests upon. Beneath the flat head of the screw, there is about 1/8" space then another flat part that is attached to this strange looking screw. It looks like a washer permanently mounted to the shaft of the screw. NOTE: YOU MAY NEED A FLASHLIGHT AT THIS STAGE TO LIGHT UNDER THE HEAD OF THE SCREW.
Now, unscrew the screw until the top of the washer comes to nearly the same level as the step in the gibb. At this point, loosen it 1/16th of a turn more, then tighten the lower screw. Your gibb should now be in adjustment.
If you see any black residue on your dovetail, that indicates premature wearing of your gibb due to lack of lubrication. You should apply 2 drops of oil on the ways on each side of the machine as the first step to every new day of work on your X3. That includes X,Y and Z axis. The gibbs should be adjusted every 3-4 weeks on average use.