mlle:
I was in this same predicament. I did have three recommenders... but one of them was being unresponsive. I was pulling my hair out, until I took a step back and realized there were other people who could write a letter for me. I was simply overlooking them because I was convinced that the mix of three recommenders I had configured was not only the ideal one; it was the only one.
Approach several of the professors with whom you have studied, and see which one seems most inclined to work with you, for example, which one asks you for bullet points around which to write your letter. (Just because you ask, does not mean you have to accept his or her offer to write a LOR.) In any case, with a 3.8 GPA, you must have some A classes on your transcripts. Start off with those professors. What about any jobs you have held? Even if you were doing something that you consider unimpressive, as long as you showed leadership, motivation, and intellectual skills--and the recommender is willing to write about these--you've got a solid recommendation letter.
Finally, and maybe I am wrong here, I think if you have two glowing recommendation letters and one that is merely good, as long as you have a solid gpa and essay and good test scores,you should be okay. Everyone (or most everyone) will have a weak spot on their application. I would think a weak third recommendation letter, as long as it is not negative, won't hurt you.