3.242. $\sqrt{(1-\sin \alpha \sin \beta)^{2}-\cos ^{2} \alpha \cos ^{2} \beta}$.
Solution.

$$
\begin{aligned}
& \sqrt{(1-\sin \alpha \sin \beta)^{2}-\cos ^{2} \alpha \cos ^{2} \beta}= \\
& =\sqrt{(1-\sin \alpha \sin \beta)^{2}-\left(1-\sin ^{2} \alpha\right)\left(1-\sin ^{2} \beta\right)}= \\
& =\sqrt{1-2 \sin \alpha \sin \beta+\sin ^{2} \alpha \sin ^{2} \beta-1+\sin ^{2} \beta+\sin ^{2} \alpha-\sin ^{2} \alpha \sin ^{2} \beta}= \\
& =\sqrt{\sin ^{2} \alpha-2 \sin \alpha \sin \beta+\sin ^{2} \beta}=\sqrt{(\sin \alpha-\sin \beta)^{2}}=|\sin \alpha-\sin \beta|
\end{aligned}
$$

Answer: $|\sin \alpha-\sin \beta|$.