A while ago NightOwl put up a two part tutorial on how to play Amazing Grace and I learned it on a 3 string. This guitar is a 3 string with an additional bass string, one octave lower than the lowest of the three normal strings. You can easily adapt any three string piece to work with the extra bass string. If anyone wants to see the amp settings there is a picture in the first (oldest) of the comments.
I'm afraid that the video is a bit over exposed. The camera was set to manual and I shot quite a bit. This was taken from later in the recording and the light levels must have risen a bit.
Because there is a lot of interest in NightOwl's excellent tutorials here are the links:
Amazing Grace cbg tutorial Part 1
Amazing Grace cbg tutorial Part 2