Each good worker can paint my new house alone in 12 hours.  Each bad worker can paint my house alone in 36 hours.  I need my house painted in 3 hours.  If I can only find 3 good workers, how many bad workers must I also find in order to have my house painted on time?
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.