Does omega 3 fatty acid supplements help a border line personality disorder person? If so how much dose? So many supplements available, how to pick?

Not for the disorder. A patient with borderline personality disorder will benefit from any health benefits provided by omega 3 fatty acids just like anyone else. Omega 3 fatty acids have not been proven to treat borderliness personality disorder but psychotherapy and treatment of any co occurring psychiatric disorders will be helpful.
It won't hurt. There's no evidence that omega 3 supplements relieve the symptoms of borderline personality disorder. However, these supplements may have benefits with regard to immune and cardiovascular function. They should be ingested with food to avoid upset stomach.
YEs. Omega 3 provides necessary nutrients for healthy balanced neurotransmitters. Its good for everybody.
They couldn't hurt. Reputable supplier & cheap. Don't take too much, it accumulates.