I just checked 3 beep is a graphic card problem. Sllt controlled? Check the graphics card and replace the power supply. the card may no longer be receiving enough power and is not being recognised. Have you already tested the graphics card?
Well, that only helps to a limited extent ^^ take the manual and look through the error codes and there are short and long tones but I assume you have 3 short ones? Do you still have the manual?