Ran into this during a slow day at the office… suggesting that the tri-fin design originated in the 1930s, and not on surfboards at all. Y’all look it over and have a think… or two.
http://www.modelwarships.com/reviews/ships/dkm/s-boat/s-100-72/72-S-100-p2.htm
There’s more about it in wikipedia (of course!) https://en.wikipedia.org/wiki/Lürssen_effect
Interesting that the original versions were used in round-bottomed hulls “which was more efficient than the more-common hard chine V-bottom”. THAT should get some amateur surfboard hydrodynamics theorists working overtime… also that the original version used trim tabs. If that isn’t enough to chew on, note also that the “cruise” setting of the outboard rudders (what we call toe-in here) was optimized at 17 degrees. Here’s a quandrary for you: based on a fin with a 5" base, 17 degrees is a toe of 1.5 inches (!) his is THAT gonna work on you stick?
If that link doesn’t work, google the Lurssen Effect with or without the umlaut over the “u”