Is the G3 a beige desktop? You need more video memory. See if theres a chip in the video slot right up next to the audio card on the board. Or install a pci video card. Look here for a chip Apple Biege G3, iMac 4MB VRAM - eBay (item 110401516023 end time Jun-18-09 14:47:00 PDT)

I have a MAG 19 inch flat panel hooked up to my beige G3.

OOOH I got it now... the Power mac {Non G3}will not lite the flat screen. There are video Dimms for them as well. Check with Find the latest Performance Upgrades, Firewire and USB Hard Drives, SATA, Memory, Laptop Battery, and more at OWC