I recently picked up a g3. It has a video card but when I connect my PC monitor to it it says I need to adjust the resolution. Problem is the screen goes black so I can't get that far in the software. It seems to boot up ok but I never get far enough to adjust the settings.
Try resetting the PRAM: at startup, hold down Command-Option-P-R all at once until the computer chimes and reboots.
That should reset the resolution to the lowest setting, letting the external screen work.