Mississippi John Hurt uses thumb, index and middle.

What is interesting though, is that you get an entirely different sound (attack) out of the strings when you switch from thumb, index and middle to just thumb and index...and therein lies a lot of the " musical intellectual heart" in the sounds of the master's Rick mentioned.

I've always been a thumb, index and middle person. Every so often, for the heck of it, I try taking tunes I've learned in this manner, and dropping the middle, just to hear the difference, and there is. To some that difference is very subtle. To others, it's a train. But it is incredibly difficult to switch between these two styles at will. It is much more work to do it with just thumb and index.

Sorry for the thread creep but fingerpicking in general is a subject near and dear to my heart.