Look in the Mach3 folder, find screen4.exe, open it . Then in screen4 go to file, open and browse to the Mach3 folder, choose the Mach3Turn.lset
Once you have the screen up double click on the DRO you want to change and you will see a box with something like %4.2f in it, change the digit after the . to a 3 so that it looks similar to %4.3f
Save then exit screen4 and open Mach turn, your DRO should now have 3 digits after the decimal.
Hood