Courtesy of BenchmarkReviews here is an extract from the article with the information you need to tackle the problem in your BIOS:
http/benchmarkreviews.com/index.php?option=com_content&task=view&id=520&Itemid=63&limit=1&limitstart=2
In their SB700 series southbridge chipsets, AMD built in a feature called the Advanced Clock Calibration (ACC). This was integrated into the BIOS by a number of motherboard manufacturers and it gave the user the ability to try and unlock the disabled cores found on their processors. With the release of the 890GX chipset and the SB850 southbridge, this feature was kept out. However, many motherboard manufacturers have still found a way allow users to attempt to unlock the disabled cores. ASUS has integrated a switch onto some of their 890GX motherboards that allows for core-unlocking, Biostar has added a feature it calls Core UnlocKING into its BIOS, and ASRock has begun releasing 890GXX boards with its UCC (Unlock CPU Cores) technology. ASRock has also stated that they have found a way to integrate this technology on to NVIDIA chipsets and will soon be using it there as well.
there was a way to look at the chip and tell if it was built on a Propus or a Deneb die. I found out that you actually can. If you take a look at the Athlon-II X3 chip, under the model number there will be a series of numbers starting with AA. If the number is AAD** AD, it means it is a Propus die. You might be able to unlock the 4th core, but definitely not the L3 cache, as there never was one. If the number is AAC** AC, it means you have a Deneb die. If this is the case, you can potentially unlock the 4th core AND the L3 cache.
Good luck and let us know how you go.