I was told that heatsinks for AM2 will work on AM3 boards, but when I followed the manual, I had this problem. The two mounting clips of the heatsink are not long enough to reach the retention brackets on the board.
Here is the pictures:
Please look at the circled area.
I think if I use force to push both mounting clips really hard, I can still make them reach the retention brackets, but I'm afraid that might hurt the CPU and the plastic retention brackets on the board will break.