Classic control theory - Negative gain required for a finite velocity error?
The transfer function \$P(s)\$ has a RHP pole at \$s=3\rm{rad/s}\$. As with any RHP pole, this means the plant is naturally unstable.

You will need to use a PD controller (transfer function \$G(s)=K_P+sK_D\$). You can select the proportional gain \$K_P\$ to achieve the desired velocity error, and select \$K_D\$ such that the zero occurs before crossover for stability (i.e., before \$|G(s)P(s)|=0\rm{db}\$).