Do you know the specific processor in the Apple G3? That would likely be very helpful. The problem is it's not just opcodes that involve the G3 it's all the specifics of the processor. PPC archetectures have changed much since 1997 (G3 era) 11 years ago is a long time technical information rots quickly on the vine. If you want to find something you need to find out SPECIFICALLY who made the PPC processor IBM or Freescale (Motorolla then), or who ever.
Try here for Freescale PPC processor line up. If you know the specific processor used I recomend you look that SPECIFIC processors part #. Next is IBM I think if you actually know where to look you will find quite a bit available but try here fore starters.
The Game Cube is PPC but it has instruction set additions. IBM made the beast.
Bottom line is you might want to look for Game Cube information regarding the opcode extensions.