not true, if set to "All" all the cores will be enabled good or not. "Auto" setting will enable all the good cores. I have an AMD x3 CPU with a bad core, I can still enable it, it just will crash the PC in any software test.
Well I never had an x3 so I wouldn't know. Either way auto or enable all cores it will unlock the fourth core if its not defective you will have 4 cores if it is defective it will crash with it enabled.