For most people, one would say that you tune a three-string dulcimer exactly like a four-string dulcimer, since a four-string dulcimer is a three-string dulcimer but with the first (nearest to you in normal playing position) string double-coursed.

Even the rare five-string dulcimer (I've seen a couple of them) is usually tuned/played as a three-stringer, but with two of the strings double-coursed.

For occasional solo use, the tuning is not particularly critical, if you can follow the dulcimer mantra - - "just find a pleasant tone."

Tuning becomes much more critical in the presence of other dulcimists, as there is some need to "find the same pleasant tone others have found." This latter imperative eludes many beginners, and is not helped by the resistance on the part of some players to finding agreement on a more or less specific pleasant tone; but usually can be achieved with practice (which apparently is not a prominent ingredient of the application for which the request first appeared).

Of course, specific and precise tuning is absolutely critical if one wishes to make a dulcimer sound like a fiddle - or guitar - but the dulcimer is of such special character that it deserves to be heard in its solo - one might say "primitive" - form, so only the wannabees (or serious performing professionals) are likely to need to move on to that kind of nonsense.

It's to enjoy.

John