"Shortfills"are nicotine free eliquid, usually sold in larger bottles.  These bottles contain enough space in the top of the bottle to top up with a nicotine containing eliquid nic-shot.

To turn your 0mg short-fill into a 3mg eliquid, simply add one 18mg nic-shot for every 50ml short-fill eliquid. Shake like your life depends on it and enjoy.