3. The smallest positive period of the function $f(x)=\sin \frac{3}{5} x+\tan \frac{3}{7} x$ is
3. $\frac{70}{3} \pi$.
The smallest positive period of $g(x)=\sin \frac{3}{5} x$ is $\frac{10}{3} \pi$, and the smallest positive period of $h(x)=\tan \frac{3}{7} \pi$ is $\frac{14}{3} \pi$. The least common multiple of 10 and 14 is 70, so the smallest positive period of $f(x)=g(x)+h(x)$ is $\frac{70}{3} \pi$.