If you are planning to run your Power Macintosh G3 headless, you should use the video adapter, part number 590-1120-A, which was included in your accessory kit. This spoofs the video card into thinking a video display is attached. When the time comes to connect a display, simply connect it to the adapter, or remove the adapter and connect the display directly to the card, as appropriate.
Be aware that if your display requires an ADB connection for full functionality, you must shut down the computer, connect the display and ADB cables, then restart the computer. A display which uses USB likely does not require a restart because USB is a hot-swappable technology.