I have a gift certificate to use at a local shop. it's about enough to cover 2 sets of control arms to compliment my 3 inch Black Diamond lift. Which ones would give me the most bang-for-the-buck; lowers, uppers, fronts or rears. Thanks.

 

front lower, followed by front upper. IMO.

It's just an opinion.
But getting your angles as close as possible to "proper" is a long term positive re the drive train.
The short distance in front makes angles much more sever so those should be addressed 1st.

I have adjustable lowers on the front and stock uppers still just due to it helped me get proper castor and I had stuffed and bent my factory driver side lower, (don;t ask).
So t was synergistic synconicity for me to get 'em at that time.
One day I will get the uppers as well but 1st I need more bullets...

 

Front lowers, you can still get about +5 to barely +6 max with the stock uppers. Then get some rear uppers for your rear drive line.