Hi D.,
I can feel your fustration.
I have a similar situation with a 5 year old ...who is quite aggressive and will fight back when being discipline. I have had a difficult time adjusting my attitude and responses to her constant crying and screaming. I believe mine is a serious case of jealousy (she has two siblings) and seeking attention, but with a very rude attitude. Her behaviour angers me so much.
Does your daughter behaves the same at school or with other care givers? Can you and your husband make any adjustment to how you respond to her tantrums? Try not to let her realize that her behaviour affects you. She will feed on that and it will just get worst. You may have already done this, but have a heart to heart talk with her. Its amazing how they see thing from their prospective and the messages they draw from our interactions with them.
Please separate you 3 year old and try to avoid her seeing this sort of behaviour and any disciplinary action you administer.
Again you are likely doing this already ...Focus on her behaviour so she understand that is what you disapprove of, not her,and do not use labels or talk about her in her presence. And from my book of error, try not to have both parents responding to the negative behaviour at the same time. She will feel attacked or question your love.
Try a kind of constant switching of the role of good cop /bad cop. But with the same message. (I'm learning this myself as both my husband and I are strict disciplinarian - it does work with some kids)
Try to highlight the positives, her good qualities and focus on than. Its easy to give more attention to the bad behaviour .. but like with anything in life, what you focus on/feed you'll get more of. And again please . . . no labels and do not tell her or have her feel anything is wrong with her.
Did she always have similar behaiour issues ?.... even before her sister was born? If no, I'd suggest that you reconsider and do away with responding with meds.
D.. all the best... I'll keep you in my prayers.
I feel your pain.
A..