I've taken all three. I scored highest on humanities (64), and since it was one of my first CLEPs, I didn't realize that I should answer all of the questions; I left about fifteen blank because I wasn't sure and assumed that there was a penalty (yes, that was a moronic thing to do). It was the easiest of the three for me. Of the two literature CLEPs, I think English Lit (61) was much easier than American lit (60), at least for me. I only scored one point better on English lit, but I studied much harder for American lit.
(Edited because I was mixing up social sciences with humanities. :-P)
(Edited because I was mixing up social sciences with humanities. :-P)