Someone actually asked this question about a week ago:
Beeping sound every 5 seconds, it won't turn on
The two Apple links I posted in my answer there are relevant here. Beeping at startup is an issue with RAM; 3 beeps indicates that the system has checked your installed RAM, but finds it unacceptable. This could mean that the RAM has physically failed; it could also mean something simple, like the RAM has gotten jostled loose in its socket(s) and now isn't seated correctly, or some bit of scuzz has gotten on the contacts/socket and there isn't a clean electrical connection.
Power On Self-Test Beep Definition - Part 2
Intel-based Mac Power On Self Test RAM error codes
Three beeps = no good banks of RAM installed. Have you removed or changed your RAM recently? Has the computer gotten knocked around?
The first thing to try is to reseat the RAM correctly:
Installing MacBook Core 2 Duo RAM
When messing with RAM, I like to use a pink pencil eraser to rub the RAM contacts clean of scuzz, and use a soft, dry toothbrush to clean the inside of the RAM socket. Then reinstall, reassemble and reboot, and see if anything changes.
If that doesn't work, the next step is to replace the RAM - preferably with sticks that you've seen working in another computer, so you can be sure they're good.
Assuming the original RAM came from Apple, it's worth contacting Apple to see if the RAM's warranteed. Normal practice in the RAM business is a lifetime replacement warranty; perhaps Apple will do that. If it's third-party RAM, definitely contact the retailer and get a replacement.
EDIT: Or wait - am I misunderstanding the problem? Does the machine still boot and run normally, aside from the beeps?