I set up my 4 string dulcimers so that the inner-most of the two melody strings is equidistant with the middle and bass, and the outermost has the same distance to the edge of the fretboard as the bass string. On a 1.5" fretboard this is usually 
7/32" | 1/16" | 1/2" | 1/2" | 7/32" with the lines representing strings.
For what it's worth, removing the outer string can put you further from the ends of the fretboard, but removing the inner string can give your thumbtip slightly more space on the melody without touching the middle string. It really doesn't matter much, unless youre like me and not very precise while playing.

You are absolutely right, Wally.  I am actually converting two of my dulcimers to 3 strings.  The one I'm working on today, I will remove the inner melody string to make the strings roughly equidistant.  On the one I've ordered strings for, I will remove the outer string.  Go figure.  Thanks!

There is no standard for string spacing. In fact there are few "standards" for anything regarding dulcimers.

In making simple capos I have found that fretboards vary in width from about 1 3/8 inch to maybe 1 3/4 inch. Usually the outer strings are about 1/4 inch from each side.

I would suggest that if you are fingering you might want 1/2 inch between the strings. For noter-drone playing, you might prefer to have the melody closer to the edge.

The simple answer is just pick a slot and you will get used to the spacing as you play.

I ordered the GHS strings yesterday from juststrings.com, but the product description didn't mention whether the bass string was wound.  If it is, I'll try your plain steel suggestion on a future re-stringing.  Thanks, John!

When converting 4 to 3, we almost always remove the outer melody string. 

If you can't find a .020 wound string bass string, just use a plain steel .020.  Traditional dulcimers were often strung that way. The lack of winding gives a subtle difference to the overall tone which adds to the 'high silvery' sound which I like.  

By eyeballing it, it looks like removing the outside string gets me there.  Just wanted to make sure.  Thanks, Dusty!

Michael, you will want to remove whichever of the double melody strings gets you closest to equidistant.  Usually that is the outside string.  But it's easy to measure or even just eyeball it.

My local music store carries single wound .020 strings.  As @jost shows, so does JustStrings.com.  Folkcraft carries both bronze wound and steel wound in that size.

If you have to go up to .022 it won't kill you (or the instrument). I use a .026 on all my standard-sized dulcimers.

Thanks, Jost!  I appreciate it!

Hello,

GHS has them: https://www.juststrings.com/ghs-d20.html?cmp=nextopia&kw=ghs-d20

Best regards, Jost.

OK, here's a follow up question.  The string information that came with my dulcimer states that the strings are standard music wire, with the bass string being an LW-20 light wound .020 diameter and the other strings are all L-12 light plain .012 diameter.  D'Addario offers .012 strings, but I don't think I've seen a bass string that is .020.  Any help on where I could find this bass string would be greatly appreciated.  Thanks.

Michael

Hey, all, 

This may sound like a "well, duh" question, but here goes.  I've been talking to a fellow dulcimer player who sings the praises of using only 3 strings.  I am about to re-string one of my dulcimers and thought I would give it a try.  So, if I want 3 equi-distant strings, I would remove and not replace the first (outside) melody string, right?  Thanks in advance.

Michael Willey