Will taking niacin, b complex and st. Johns wort everyday help my anxiety?

Help anxiety. Yes and no. Many patients have the benefits using all was mentioned. I would like to recommend check causes of your anxiety, determine any general medical condition, and if all is fine, those could help you, also add yoga, meditation, acupuncture, good nutrition, psychotherapy, etc.
Not how you think. It's unlikely that any or all will be in themselves materially important there is no evidence they are reliably "anxiolytic" (ie, anti-anxiety). But if you believe them to be empowering, that may facilitate a positive self-fulfilling prophecy and enable you to make other attitudinal and lifestyle changes that do improve anxiety. Be reasonable, but do take charge of your mental health.