The other Orthey dulcimers I've seen had VSLs of 27". If yours is close to that, a wound bass string around .024 would probably be right. Middle string perhaps .012 or so and the melody string would depend on your tuning. If you are tuning DAA, then keep a .012 for the melody. If DAd, perhaps .010.
My personal preference would be for slightly heavier strings (.026, .014, .012) but those that I list might be close to standard for that size dulcimer.
And as the image suggests, you will need loop end strings, although if all you can get are ball end, you can always remove the ball manually. Just squeeze them gently with plyers and pop them out of the strings.
Dusty T., Northern California
As a musician, you have to keep one foot back in the past and one foot forward into the future.
-- Dizzy Gillespie