That's pretty wild... I'll have to ask around about three-string tunings, but I almost bet they are modal, or octaves, and designed to be played like a dulcimer.
Aquila does make a set of true gut strings for banjo, so they may well have been. They are a tuning nightmare in outdoor conditions where the humidity fluctuates though.
I had a fun gig for a season in a living history scene, a civil war era, period social band called the StringTyme Minstrels. I played guitar/mandolin in that one... had to wear a wool suit in July, and be part of a little Henrico County encampment. In the evening we played for a big dance. That was great fun.
