Your computer will receive an irregular POST if it something wrong with it's hardware. An irregular POST is a beep code that is different from the standard one or two beeps.

Here is the semnification of the 3 beeps you receive regarding what BIOS version do you have:

AWARD BIOS

1 long, 2 short beeps indicates a video error has occurred and the BIOS cannot initialize the video screen to display any information. Click here for the solution.

IBM BIOS:

Three Long Beeps means a keyboard or keyboard card error.Go to this link for the solution.

One Long and Two Short Beeps means that you have a video (Mono/CGA Display Circuitry) issue. Check here for how to solve it.

AMI Bios

3 short beeps means Base 64K RAM failure. Check here for the solution

PS: If you opened your computer recently try to checked for losed cables or computer components that does not fit well in their slot.