>>537796453
Correct. The Crown was placed upon a elder brain, at which point the three Chosen were able to use the Crown's intended purpose of controlling the wearer to control the elder brain.

After the elder brain wearing the crown had been exposed to that Netherese magic for an extended period of time, it underwent a magical evolution into a Netherbrain, a Netherse infused elder brain.

At was at this time that the Crowns control over the brain was also destabilized by the murder of Ketheric Thorme. With only two stones still active, the crown lost some integrity, and the Netherbrain was powerful enough to begin busking control of the Crown.

So yes, there are solid explanations why the stones from one person could not control a Netherbrain, after they had previously been able to control a normal elder brain under the efforts of three people.