There is no Emac G3. You either have an iMac G3, or an eMac G4.
If you have an iMac G3, 10.4.11 is the maximum OS that you can run. If you have an eMac, and your processor speed is over 867MHz, you can run 10.5 "Leopard"
Snow Leopard will not run on either, as both are PowerPC computers.