3. A water tank is equipped with 9 inlet and outlet pipes numbered $1,2, \cdots, 9$. Some are only for inlet, and some are only for outlet. It is known that the pipe numbers opened and the time required to fill the tank are as shown in Table 1. If all 9 pipes are opened at the same time, the time required to fill the tank is ( ) hours.
(A) 1
(B) 2
(C) 3
(D) 4
Table 1
\begin{tabular}{|c|c|c|c|c|c|c|c|c|c|}
\hline Pipe Number & 1,2 & 2,3 & 3,4 & 4,5 & 5,6 & 6,7 & 7,8 & 8,9 & 9,1 \\
\hline Time (hours) & 2 & 4 & 8 & 16 & 31 & 62 & 124 & 248 & 496 \\
\hline
\end{tabular}
3.B.

Let it take $t$ hours to fill the pool, then

i.e., $\left[\frac{\frac{1}{2}\left(1-\frac{1}{16}\right)}{1-\frac{1}{2}}+\frac{\frac{1}{31}\left(1-\frac{1}{32}\right)}{1-\frac{1}{2}}\right] t=2$.
Solving for $t$ gives $t=2$.