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 $S_{\triangle A B C}=$ $\qquad$
3.4.

According to the problem,
$$
S_{\triangle A B C}=2 \times 7-\frac{1}{2}-\frac{7}{2}-6=4 \text {. }
$$