3. In the complex plane, the complex numbers $3-\mathrm{i}, 2-2 \mathrm{i}, 1+5 \mathrm{i}$ correspond to points $A, B, C$, respectively, then the area of $\triangle A B C$ is
$$
S_{\triangle A B C}=S_{\triangle O A B}+S_{\triangle O A C}-S_{\triangle O B C}=\frac{1}{2}(|-6+2|+|15+1|-|10+2|)=4 .
$$

The translation is as follows:

$$
S_{\triangle A B C}=S_{\triangle O A B}+S_{\triangle O A C}-S_{\triangle O B C}=\frac{1}{2}(|-6+2|+|15+1|-|10+2|)=4 .
$$