For general information, beeps are used to diagnose a variety of RAM and EFI fault conditions in Intel Macs. Apple's support note lists the beep codes and the cases:
Mac computers: About startup tones
3 beeps - pause - repeat means that the RAM did not pass the startup integrity check. This can happen for a number of reasons: Bad RAM, incorrect speed, bad installation, gunk in the RAM sockets, broken contacts. The first step is to follow Dan's wise suggestions to check the RAM type for compatibility, clean the sockets and the RAM contacts, and reinstall.
If those steps don't solve the problem, it's possible that the RAM itself is bad. One way of checking RAM integrity is by running Apple Hardware Test, a testing utility that bypasses the system software to check the hardware underneath, including the RAM. Machines like yours, which shipped with OSX 10.7 or later, have a hidden partition on the hard drive with AHT pre-installed. If you hold down the D key as you press the Power button, and keep holding it, you should boot into AHT, which will allow you to run a RAM test. Here's Apple's description of the process:
Using Apple Hardware Test
These days, most RAM carries a lifetime warranty. If the original RAM is having problems, Apple should replace the sticks, even if the computer is out of warranty; check with your local Apple Store. If the RAM is from a third-party retailer, that retailer should replace the defective RAM.