Note positions on a Sanshin neck (as taken from a little book I have from Japan).
Open stings, 22mm, 64mm, 93mm, 122mm, 150mm, 173mm, 198mm, 218mm, 237mm, 265mm, 280mm, 300mm
Take note that in traditional Sanshin playing the only notes used are as follows: Open strings all three strings, 64mm all three strings, 122mm all three strings, 150mm all three strings, 173mm all three strings, 198mm all three strings, 237mm center and thin string only, 280mm center string only, and 300mm center and thin string only.
In a pinch nylon fishing line can be used for stings on a Sanshin but I have never heard details about specific sizes. Remember that the original strings for the Karkarna Sanshin were old parachute cords.
How any or all of this relates to playing on a real Shamisen I have no clue, so I will leave that up to Kyle. I just hope it helps since for now I only have a very nice Sanshin and it will be some time until I can upgrade to a Shamisen of my own.