Loz – dont beat yourself up about it, youre bound to relapse at some point, its a normal part of recovery. Get back on track – I would personally avoid going out and drinking. Detach yourself from anyone/anywhere that potentially could cause you to relapse.
You went 3 weeks without it which is amazing.. you can do this, go to the doctors and get some help and go to support groups thatll help.
Get a hobby, read, go to the gym – avoid social situations where coke is likely to be present – just until you feel you have control.
Good luck x