I have also trialled low dose anti-psychotics in the past (risperidone and abilify) and they did not seem to make any significant improvement in my stutter. In fact they potentially made my stuttering even worse. I have heard some people on this forum claim massive improvements while on anti-psychotics but I guess they work for some people and not others. Personally I wouldn't keep chasing the potential of anti-psychotics if you have already tried three and they all proved ineffective.