Be successful using computers? Pull the other one! (4,3,2)
I believe the answer is:
come off it
'pull the other one' is the definition.
(both are expressions of incredulity)
'be successful using computers?' is the wordplay.
'be successful' becomes 'come off' (if something comes off, it works as planned).
'using computers?' becomes 'IT' (information technology).
'comeoff'+'it'='COME OFF IT!'
(Other definitions for come off it that I've seen before include "You're mad!" , "stop fooling" , "How to stop sex addiction?" , "Stop messing about!" , "Drug addict should" .)