This is what it says on Wikipedia:-

The Three Stringed Trance Wonder

    * This is a normal guitar, but with only three strings. It has an old Harmony pickup added (with duct tape) and is played tuned to G, G and B using an E string in the A position, a D in the G position and a G in the B position. He often tells the story that he bought it for $75 in this condition in Como, Mississippi from a man named Sherman Cooper, and vowed never to add another string, and that he would tour the world telling his story of how Sherman ripped him off. He has since done this, telling the story at all his gigs. He also says at his gigs (while picking up or putting away the guitar) that it is the "...biggest piece of shit in the world, I swear".