Answering slow Sicilian set up (including Bc4) as Black I usually answer 1. e4 with the Sicilian, then after 2. Nf3 I play 2...d6 hoping for a Najdorf or other open Sicilian lines. I have quite a hard time though with slow set up like 3. Bc4 followed by d3 and Nf3 (not necessarily in this order). I tend to lose against these set up (I just find them boring). 
What plan would you recommend for Black? I feel like refusing to play d4 early in the game should be punished but I don't see how. Thanks in advance!
I don't think that 3.Bc4 is so bad that Black should get an advantage. I believe that Adams played it several times with decent results, and it's not the worst way to avoid deep theory. But Black has easy equality.
You can play ..a6 (to which he will usually play a4) and ..e6. What then is that Bishop doing? Having played against it once or twice I agree that it's boring but there is not much you can do about it.