A 50ml shortfill will be a 60ml shortfill bottle size and a 100ml shortfill will be a 120ml shortfill bottle size. This is to allow 10ml or 20ml of nicotine to be added.

By adding 1 x 18mg nicotine shot into a 50ml shortfill you will achieve the equivalent of a 3mg e-liquid.
By adding 2 x 18mg nicotine shots into a 100ml shortfill you will achieve the equivalent of a 3mg e-liquid.