yuiopao:##yuiopao:14975:Alex Vart student:/home/stud/yuiopao:/bin/bash
   polkovar:##polkovar:5004:John Adi student:/home/stud/polkovar:/bin/bash
   gfgfdgd:##gfgfdgd:11000:Alan Perk student:/home/stud/gfgfdgd:/bin/bash

grep '^.*:.*:[1-9][0-9]{3}:'

grep '^[^:]*:[^:]*:[1-9][0-9]{3}:'