If it says "Power Macintosh G3" on the computer and has a built in monitor, it's known as a "Power Macintosh G3 All in One." If the flashing disk has a question mark, the computer cannot find an operating system to boot. This can be as simple as the disk being blank, but could certainly also be a disk failure as you suspect. The flickering screen is most likely a monitor or power supply issue. Does the picture on the screen distort or change color or does it just flash off and on? There's a chance the video cable could be loose inside. To confirm the model is correct, visit everymac.com and look for your computer. Once you find it, you'll find all the specs including which versions of the OS work with your machine.