I've recently bought this Power Mac G4 on eBay.
When powering on - all I get were 3 beeps. There was no video output.
So - I've been googling this issue and found one page with information about the beep codes: http://www.welovemacs.com/whdoallthbme.html
Obviously there was a problem with one or more RAM modules. Therefore I've been switching the modules and discovered that 1 module seemed to be broken. After removing the broken one the computer was booting up as normal.