First find their rates...
1/6, 1/2, and 1/3 now combined they will perform the job in one day if:
t/6+t/2+t/3=1 make all have a common denominator
t/6+3t/6+2t/6=6/6 which is equal to:
6t=6 so
t=1
So if they all work together they will complete the job in one day.