Personally, I only had two.
I had gone abroad for a year, and when I came back, I only had one semester until graduation. Then, on top of that, I didn't apply until I had been out of school for a year... Talk about difficulty of getting profs to remember you... (I didn't want to ask my employer because I didn't want my employer to know that I was applying; didn't want them to think about replacing me before I even had somewhere to go, you know?)
Anyway, I only had two, and I still did pretty well. (You can see my stats at LSN, linked below). On the other hand, I went to a pretty respected undergrad (Berkeley) and the two recs I did have were from Lecturers in my major department.
I made sure to give them copies of my transcript, my resume, and a copy of my rough draft personal statement so that they could comment on the same qualities I was trying to highlight. I also spent hours corresponding with them via email regarding my intentions behind going to law school, and one of my professors even asked if there was something I would like her to highlight in particular. (I told her, "I don't typically do as well on standardized tests as I do in real life, so if you could emphasize that, that would be great." Considering that I got into two Top 10 programs and waitlisted at another 3 with only a 161 LSAT, I suppose she did.)
In the end, I think having the letters two was enough. Nonetheless, if I could have, I would have gone for the third in a heartbeat. I just didn't know anybody anymore who could really give me one...
My advice: if you can get three, great, but if the third one would be merely mediocre, go with the two letters of quality.
DO NOT go with just one. I think THAT would be virtual suicide... (It's not hard to find one person who thinks you're great; it's a lot harder to find two or three, y'know? And remember, they prefer letters from Professors because you are applying to law school, not for a job as a lawyer...)