Bass dulcimer strings
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                    Sorry Bill, I mis-wrote that tuning below...   A Bass Dulcimer 1-3-5 tuning is designated D'A'D or C'G'C. 
Notice the ' marks on the first D and A.   This tells us that those notes are a full octave lower than the common DAd. 
In fact, on a bass dulcimer the melody string D is the same as the bass string D on a regular dulcimer.   If you look on the Strothers Calculator that I posted below, you will find that for your very short VSL Bass Dulcimer you will need the following strings:
D' = .035
A' = .027
D = .020
You won't be able to put those strings on a regular dulcimer nut and bridge -- the slots will be too narrow.  If you're trying to make a bass dulcimer out of a regular dulcimer, you will need to open up the string slots on the nut and bridge, with a small triangular file, so the strings can sit properly. Not particularly deeper, but wider.
Most bass dulcimers that I have seen over the years have had much longer VSLs -- 28", 29" even 30".