Recently got an X3 720 and I wanted to attempt to unlock the 4th core. I did the normal procedure of setting ACC to auto in the BIOS, but when I rebooted there was absolutely no effect - this also happened when trying the "all cores" setting. ACC remains on but it doesn't seem to be causing the 4th core to show. My motherboard is an MSI 770-C45.
You might just be unlucky. If you got it from Newegg like me, chances are you got one of the more difficult ones. You can try to manually unlock the 4th by putting the ACC setting to "per core" and turn the Value of the 4th core to -6% and see how it goes. Put cores 1-3 at -2%. Worked for me.
Once that works, lower the 4th core to just -4%. I was able to do that on mine, but couldn't get to -2% on the 4th core, or 0% on any of the other cores.
Then you can't unlock the 4th core. Most of the CPUs that get turned into X3s have a defective core. The ones that are unlock-able are just slightly defective and usually need increased voltages and tolerances. Be happy with your 3 cores. If you wanted a quad core then buy a quad core. Unlocking cores is never guaranteed, it's just a nice bonus. Also, only certain boards have the old microcode that lets them try to unlock cores, and I don't think your's is one of them.
I didn't buy the processor with the intention of converting it to a quad core and knew it probably wouldn't work - I just thought I might as well try. If my board doesn't have the code needed to unlock cores, could a flash to an older BIOS rectify this?
By no effect, did you mean you were able to boot into the desktop and do stuff? If so then you might also need to turn on ECS hydrid mode on. I know thats what I had to do to get the 4th core, I have GA-MA790XT mobo.