Two of the people were using mental health services at the time of the deaths, and the other had been discharged as a patient.
Mental health bosses have not revealed any details about the patients involved, or the deaths – which were among the 13 total homicides in Notts between April 2009 and March 2010.
Professor Peter Bartlett of the University of Nottingham, an expert in mental health law, said: “The easy response is ‘why wasn’t the person locked up’, but how dangerous someone is can be hard to predict. Even with the best predictors we have now, to make a significant impact on homicide rates, you end up locking up far more people who won’t be violent.”
A spokesman for Notts Healthcare Trust, which runs mental health services, said the incidents were subject to “the highest level of investigation”.
But medical director Dr Peter Miller said: “The number of homicides committed by people engaged in mental health services is very low. In fact, our service users are far more likely to be the victims of violence than the perpetrators.”
Jackie Valad of Arnold, whose daughter Elizabeth was killed by a psychiatric patient, said the rights of victims’ families were not being put highly enough.
She said: “It just goes on and on. Something has got to be done.”