The stock heatsink included with the i3-2100 is not going to fit that Phenom processor. Your Phenom processor should have come with a stock heatsink.
The Phenom X4 9600 fits an AM2+ socket. The i3-2100 fits an LGA 1155 socket. While you can't see it on the Wikipedia page, the LGA 1155 heatsink is held down by four pins that fit in holes in the motherboard. The AM2+ heatsink is held down by hooks that attach to connection points on a plastic ring around the socket. You aren't going to find the holes that the LGA 1155 heatsink needs on an AM2+ motherboard.
While there are universal heatsinks, the stock heatsink that comes with the i3 isn't one of them. Universal heatsinks usually come with backplates and other specialized hardware to allow them to attach properly.