G3 kernel is needed to boot into the installer environment on powerbook g3.
The G4 kernel is used to boot a G3 :-) The G3 bootoption in the yaboot bootloader should load the G4 kernel correctly in the 2004.1 release. The initial 2004.0 release bootloader was misconfigured and tried loading a non-existent G3 kernel (the G4 kernel provides G3 support). This bug was fixed as soon as a day after the 2004.0 release and new isos were put online. The -infra structure team has been ignoring my requests to update the files on the mirrors. I stopped asking to update the 2004.0 release when 2004.1 was released.