To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $2mn$ and $6m^{2}$ is $6nm^{2}$. Multiply $\frac{3}{2mn}$ times $\frac{3m}{3m}$. Multiply $\frac{n}{6m^{2}}$ times $\frac{n}{n}$.
$$\frac{3\times 3m}{6nm^{2}}+\frac{nn}{6nm^{2}}$$
Since $\frac{3\times 3m}{6nm^{2}}$ and $\frac{nn}{6nm^{2}}$ have the same denominator, add them by adding their numerators.
