As Gary mentioned, the easiest solution is to use an older 4:3 aspect ratio display. Many of the older LCD displays make their way to thrift stores through donations. If you'd like to use a modern widescreen display, there is an alternative - but it involves investing $$ in your G3. If you upgraded your G3 tower to Mac OS 9.1 (and ran the OS 9.2.1 and OS 9.2.2 Updates), you could install an original Radeon Mac Edition or Radeon 7000 PCI graphics card (32 MBs of DDR memory) in one of the available PCI slots. This 2001-vintage card provides support for a widescreen display. Unfortunately, OS 8.6 doesn't support the Radeon driver package. If interested, you should check eBay for the OS 9 (retail/universal installer version) software and the Radeon graphics card, which must be designed for use in a Mac, meaning that it's programmed with a Mac ROM. The Radeon 7000 PCI card for PCs cannot be used, unless its ROM has been reprogrammed. The beige G3 has (3) PCI slots and the Blue & White G3 has a designated 66 MHz PCI slot for the graphics card. The B&W's original card - the Apple/ATI Rage 128 - doesn't support the native resolution/refresh rate of a typical widescreen display. I have Radeon cards in a pair of my B&W G3s, enabling use of a 19" Dell widescreen LCD (native resolution: 1440x900 @ 60Hz).
Which model G3 do you have? A Beige MiniTower or a Blue&White Tower? Both could be running OS 8.6... so not enough info in your post to give a definitive answer.
The older beige colored versions (called minitowers, though they were pretty full-sized towers) have a Macintosh video port that requires an adapter to fit a standard VGA monitor, some of these have little switches on them to set the resolution. When I first bought my Viewsonic monitor for my Beige G3 in 1998 it came with the Macinstosh adapter, but I don't know if you can find those easily now. The adapter is required if you are using the original built-in video card. PCI video cards from that era for the G3s could add a VGA and DVI port to give you additional multiple-display options.
The later Blue and White G3 towers have a standard VGA video port. Though the newer DVI and HDMI are more common on new monitors you should be able to find a VGA port option on many current or used displays.