A Dobson unit (DU) is the most common unit of measurement used in ozone
research. The unit is named after G.M.B. Dobson (1920-1960), one of the first scientists to investigate atmospheric ozone.
Basically, a Dobson unit is equivalent to 0.01 mm thick pure ozone if you brought the ozone down to ground level pressure (1 atm) and 273K temperature. 1 DU = 1 matm cm.
So, a 3 mm thick slab of ozone at STP
corresponds to 300 DU, which is approximately the current global average for ozone thickness.