Work done by A in one day = 1/14 part of work.
Work done by B in one day = 1/21 part of work.
Work done by (A+B) in one day = 1/14 + 1/21 = 5/42 part of work
Work done by (A+B) in three days = 15/42 part of work
Remainimg work = 1 - 15/42 = 27/42 = 9/14 part of work
Now the remaining work 9/14 is done by B alone.
1/21 of work done by B = one day
9/14 of work done by B = 9/14/(1/21) = 9x21/14 = 27/2 days = 13.5 days
Total time to complete this work = 13.5 + 3 = 16.5 days