The key feature shared by the 3 disorders are the schizophrenic symptoms, which are delusions, hallucinations, or grossly disorganized or catatonic behavior, language disorder, disorganized speech, and social isolation.
But the three illnesses differ in the duration of symptoms:
In schizophrenia, it is six months or more,
Schizophreniform disorder last at least a month but less than six months.
And brief psychotic disease express one of the schizophrenic symptoms, and last few days but less than one month.