3. Given the complex number $z=\cos \frac{2 \pi}{3}+\mathrm{i} \sin \frac{2 \pi}{3}$, where $\mathrm{i}$ is the imaginary unit, then $z^{3}+z^{2}=$
$$
z^{3}+z^{2}=1+\cos \frac{4 \pi}{3}+\mathrm{i} \sin \frac{4 \pi}{3}=\frac{1-\sqrt{3} \mathrm{i}}{2} .
$$