A and B are opened on the empty tank, and after 4 hours C is opened too.

Explanation

1. 01

   In 4 hours A and B fill $4 \times \frac{5}{36} = \frac{5}{9}$, leaving $\frac{4}{9}$.

2. 02

   With all three open the net rate is $\frac{1}{36}$, so $\frac{4}{9} \times 36 = 16$ hours.