Posted 01 March 2011 - 11:52 AM
Three beeps is a RAM failure. No point trying to test your RAM, you won't be able to boot off the OS DVD as the RAM is fried.
10 years old so it's a G3.
G3 iMacs have a cover on the bottom that will let you get to the RAM. You will only need a coin to turn the cover to get it off. Get at the RAM, pop it out and reseat it, try to start up again. If it doesn't boot, try popping out one of the RAM strips and try again, if it still won't boot swap them over and try again.
Hopefully you will get the Mac to boot with a single strip of RAM, this will allow you to limp along until you can get another strip.
If you only have a single strip of RAM you are sol. Go and buy some more.
512MB of RAM for a G3 is cheap (about £15($20) or so) The G3 can take up to 1GB in 2x 512MB strips.
Pre 1999 G3's don't have the RAM cover, it's a pain to get at the RAM so you got lucky!