jmc

Each good worker can paint $1/12$ of my house in an hour, so three of them together can paint $3/12=1/4$ of my house in an hour. So, in 3 hours, the three good workers will paint $3(1/4)=3/4$ of my house. The bad workers have to paint the other $1/4$ of the house. Each bad worker paints $1/36$ of the house in an hour, so each bad worker can paint $3(1/36)=1/12$ of the house in three hours. Since the bad workers together need to paint $1/4$ of the house, and each bad worker can paint $1/12$ of the house in three hours, I need $(1/4)/(1/12)=\boxed{3}$ bad workers.