To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $\left(x+3\right)^{2}$ and $x+3$ is $\left(x+3\right)^{2}$. Multiply $\frac{1}{x+3}$ times $\frac{x+3}{x+3}$.
Since $\frac{3}{\left(x+3\right)^{2}}$ and $\frac{x+3}{\left(x+3\right)^{2}}$ have the same denominator, subtract them by subtracting their numerators.
