If you notice the condition in the question, you will observe that there is a circular misplacement. By which I mean that, if Black is wrongly labeled as Black, Black cannot be labeled as White. So, it must be named as Back + White. If you consider that all the 3 jars are wrongly placed, that is, Black + White jar contains either the Black balls or the White balls, but not the both. Now, just assume you pick one ball from the Black + White jar and let us assume it to be a Black ball. So, obviously, you will name the jar as Black. However, the jar labeled Black cannot have Black + White. Thus, the third jar left in the process should be labeled Black + White. So, if you just pick up one ball, you can correctly label the jars.