#### Answer

$m \angle 3 = 120$

#### Work Step by Step

Complementary angles are angles that add up to $90^{\circ}$. Supplementary angles are those that add up to $180^{\circ}$.
If $\angle 1$ and $\angle 2$ are complementary, then they add up to $90^{\circ}$. If $m \angle 2$ is $30^{\circ}$, then we find $m \angle 1$ by the following equation:
$m \angle 1 = 90 - m \angle 2$
Let's plug in what we know:
$m \angle 1 = 90 - 30$
Subtract to solve:
$m \angle 1 = 60$
If we know that $\angle 1$ is supplementary to $\angle 3$, then we can use the following equation to determine $m \angle 3$:
$m \angle 3 = 180 - m \angle 1$
Let's plug in our value for $m \angle 1$:
$m \angle 3 = 180 - 60$
Subtract to solve:
$m \angle 3 = 120$