|It appears that you have an AMI BIOS. See your beep codes here:|
If you are sure that it is 3 short beep codes then:
Your computer has memory problems. First check video. If video is working, you'll see an error message. If not, you have a parity error in your first 64K of memory. First check your SIMM's. Reseat them and reboot. If this doesn't do it, the memory chips may be bad. You can try switching the first and second banks memory chips. First banks are the memory banks that your CPU finds its first 64K of base memory in. You'll need to consult your manual to see which bank is first. If all your memory tests good, you probably need to buy another motherboard.
NOTE: You have not told us what memory speed you are using only that it is DDR3. Your manual has cautionary notes against memory useage. Read those carefully. It also stipulates certain conditions on use of dual channel memory. Read those also. It also stipulates relationship of your memory with the CPU. Make sure that is not a problem.
Check that there isn't a mounting post or something like that possibly shorting the motherboard underneath.
If you have not tried it, then I suggest use a single stick of memory in single channel mode and see if that works.
If it is video and its on board, then I suggest you go back to your seller and either get a replacement or a refund.
☺ When everything else fails, read the instructions.