There are plenty of three-string dulcimers around (I own one, which I made in 1972 at the Fox Hollow Festival with the help of the folk from Minnesota-based Here, Inc.), and plenty with four. (Many of the four-stringed variety just have a doubled-up 1st string to give more oomph to the melody line.) The very best, simplest, most elegant explanation of Appalachian dulcimers (or mountain dulcimers, or lap dulcimers) and their strings, tuning, and playing is "The Dulcimer Book" by Jean Ritchie. It's still in print, and a great resource. As is Jean, herself, by the way. See the related thread on a recent Irish Times article about Jean -- good reading (and sorry for thread-creep!). Kathy