They can still charge her with the other two deaths. However, without some new punishment evidence (particularly regarding future dangerousness), they will not be able to seek the death penalty because of a doctrine known as collateral estoppel (much like double jeaopardy but issue-based rather than offense-based).
As someone experienced in capital litigation, I can tell this sentence is not surprising. There just was not much evidence showing a continuing threat to society and the defense put on a good mitigating circumstances case. All along, I felt that the insanity defense was simply an early attempt to plant the seeds of mitigation evidence in the jury's head.