jmc

The number of people mowing and the time required to mow are inversely proportional. Letting $n$ be the number of people and $t$ be the amount of time, we have $nt=(5)(12)=60$ because 5 people can mow a lawn in 12 hours. If $m$ people can mow the lawn in 3 hours, then we must have $m(3)=60$, so $m=20$. Therefore, we need to add $20-5=\boxed{15}$ people to the job.