#### Answer

$3m$

#### Work Step by Step

The greatest common factor of a group of terms is the greatest factor that evenly divides into all terms in the group.
In this case, $3m$ is the greatest common factor, because it is the largest factor that divides evenly into all 3 terms.
$9m^{3}\div3m=3m^{2}$
$3m^{2}\div3m=m$
$15m\div3m=5$