Apple has a variety of 'beeps' (sound) and 'blinks' (lights) for both computers and monitors. Your 3 beeps (wild guessing here) could be as noted in this link below ... which will also steer you in the right direction. Note too, that blinking lights - can have a pattern (3 blinks: short-short-long, for example). These also have specific diagnostic meanings and you might make a note before searching.
Link to some ref on blinks and beeps is here (a paste follows this note too):
G5 mac reference - PASTED DIRECTLY FROM THE APPLE COMMUNITY SUPPORT PAGE AT THE LINK ABOVE:
Sep 26, 2012 9:00 AM
in response to Kort
' This helped me
For future reference:
Power-On Self-Test Beep Definition - Part 1 - http://support.apple.com/kb/TA43884
Power On Self-Test Beep Definition - Part 2 - http://www.info.apple.com/kbnum/n58442
The power on self-test resides in the ROM of the computer. This test runs whenever the computer is turned on after being fully shut down (the power-on self-test does not run if the computer is only restarted).
If a fault is detected during the test, you will not hear a normal startup chime. Instead, the system will beep as explained below. If you experience one of these beeps, you may call your Apple Authorized Service Provider for additional troubleshooting assistance.
1 beep = no RAM installed
2 beeps = incompatible RAM types
3 beeps = no good banks
4 beeps = no good boot images in the boot ROM (and/or bad sys config block)
5 beeps = processor is not usable
In addition to the beeps, on some computers the power LED will flash a corresponding number of times plus one. The LED will repeat the sequence after approximately a 5-second pause. The tones are only played once.
You can also run Apple Hardware test from your original installer disc. Sometimes with RAM it is just easier to pull units and test them individually.