3. In astronomy, "parsec" is commonly used as a unit of distance. If in a right triangle $\triangle ABC$,
$$
\angle ACB=90^{\circ}, CB=1.496 \times 10^{8} \text{ km},
$$

i.e., the length of side $CB$ is equal to the average distance between the Moon $(C)$ and the Earth $(B)$, then, when the size of $\angle BAC$ is 1 second (1 degree equals 60 minutes, 1 minute equals 60 seconds), the length of the hypotenuse $AB$ is 1 parsec. Therefore, 1 parsec = $\qquad$ km (expressed in scientific notation, retaining four significant figures).
$\begin{array}{l}\text { 3. } 3.086 \times \\ 10^{13}\end{array}$