Doing it at work? (2,3,3)
I believe the answer is:
on the job
I believe this is a double definition.
'doing it' is the first definition.
(both can mean having sex)
'at work?' is the second definition.
(I've seen this before)
(Other definitions for on the job that I've seen before include "At work" , "Informally working, being busy" , "Bonking" , "(Of training) done while working" .)