Not possible. Leopard requires "Velocity Engine" that only exist in G4s. To be honest, theres really no point trying to get Leopard on G3, its already dead slow. Even G4 is struggling to run Leopard properly
There are some G4 upgrades for the iMac G3. It won't allow you to normally install Leopard, but /maybe/ if you are able to hack 10.5.3 onto there, a 500 MHz G4 + 1 GB RAM + a fast HDD might get you something somewhat usable on 10.5. The graphics will probably end up being terrible for Leopard. Also, use XSlimmer. You'll probably have to do that to make it usable at all, and even then you probably couldn't do much other than experiment with the OS. It might not be completely unusable, I've even heard stories of getting decent experience on a 400 MHz TiBook.