3. If the multiplicative function $f(n)$ is defined at $n=-1$, then $f(-1)= \pm 1$.


3. $(f(-1))^{2}=f((-1) \cdot(-1))=f(1)=1$.

The translation is as follows:

3. $(f(-1))^{2}=f((-1) \cdot(-1))=f(1)=1$.